The development and study of titanium dioxide based buoyant composite photocatalyts for improved applications in photocatalytic degradation of organic pollutants in aqueous solutions

THE DEVELOPMENT AND STUDY OF TITANIUM DIOXIDE BASED BUOYANT COMPOSITE PHOTOCATALYSTS FOR IMPROVED APPLICATIONS IN PHOTOCATALYTIC DEGRADATION OF ORGANIC POLLUTANTS IN AQUEOUS SOLUTIONS HAN HUI NATIONAL UNIVERSITY OF SINGAPORE 2012 THE DEVELOPMENT AND STUDY OF TITANIUM DIOXIDE BASED BUOYANT COMPOSITE PHOTOCATALYSTS FOR IMPROVED APPLICATIONS IN PHOTOCATALYTIC DEGRADATION OF ORGANIC POLLUTANTS IN AQUEOUS SOLUTIONS HAN HUI (M Eng., Dalian Maritime University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2012 Acknowledgement First of all, I would like to thank my supervisor Prof Bai Renbi, who is courageous to support this project financially and spiritually from the beginning Prof Bai guided me all along, supported me when I was down and corrected me when I was wrong I have learned a lot from him, not only about doing research, but also about being a researcher Without his wisdom and endurance to me, it is really not possible to finish this thesis I would also like to express my appreciation to all the group members in particular Dr Li Nan, Dr Liu Changkun, Dr Wee Kin Ho, Dr He Yi, Dr Han Wei, Ms Tu Wenting, Ms Zhang Linzi and Mr Zhu Xiaoying Over the past years, we have grown together and I indeed enjoyed working with them My thanks also go out to our technicians Ms Susan, Ms Mary, Ms Hwee Bee, Mr Suki and Mr Sidek who had helped me a lot throughout the work In addition, I would also appreciate the assistance and cooperation of the Final Year Project students Ms Yeong Sok Ming, Ms Ng Pei Shi Patryce and Ms Sun Chenxi Finally, heartful thanks go to my family and friends for their immense support and love along the way I Table of Contents Acknowledgement I Table of Contents II Summary V List of Tables XI List of Figures XII List of Symbols XVI Chapter Introduction 1.1 Overview 1.2 Research objectives and scopes 11 1.3 Organization of the thesis 13 Chapter Literature Review 15 2.1 TiO2 photocatalyst 15 2.1.1 TiO2 crystal structures 15 2.1.2 Precious metal deposition on TiO2 17 2.2 Modifications of TiO2 20 2.2.1 Semiconductor combined TiO2 20 2.2.2 Metal ion doped TiO2 22 2.2.3 Sensitized TiO2 25 2.2.4 Non-metal doped TiO2 27 2.3 Buoyant photocatalyst substrates 31 2.4 Preparation of modified TiO2 on polymer substrates at low temperature 36 2.5 TiO2 configuration effect on photocatalytic reaction 39 2.6 Photocatalytic reactor engineering 42 2.6.1 Slurry system and immobilized photocatalyst reactors 42 2.6.2 Combined with other processes 43 2.6.3 Photocatalytic reactors using solar light 45 2.6.4 Buoyant photocatalyst processes 47 2.7 Remarks 50 Chapter Development of a Buoyant Composite Photocatalyst with Visible Light Activity Using a Low Temperature Hydrothermal Method 52 3.1 Introduction 52 3.2 Experimental 55 3.2.1 Material preparation 55 3.2.2 Material characterizations 56 3.2.3 Photocatalytic activity tests 58 3.3 Results and discussion 61 3.3.1 Morphologies of prepared photocatalysts 61 II 3.3.2 Crystalline structures and compositions of prepared photocatalysts 62 3.3.3 XPS studies 65 3.3.4 FTIR analysis 69 3.3.5 UV-Vis absorption spectra 70 3.3.6 Photocatalytic oxidation activity 74 3.3.7 Effect of TEA treatment time on light activity 77 3.4 Conclusions 81 Chapter Preparation of Buoyant Composite Photocatalyst with High Photocatalyst 82 Loading through a Novel Layered Rutile and Anatase TiO2 Configuration 4.1 Introduction 82 4.2 Experimental 84 4.2.1 Preparation of buoyant composite photocatalysts with a layered-TiO2 configuration 84 4.2.2 Characterization of prepared buoyant composite photocatalysts 86 4.2.3 Degradation of MO dye in aqueous solutions by the prepared photocatalysts 87 4.3 Results and discussion 89 4.3.1 XPS spectra of PPF substrate 89 4.3.2 Surface morphology and elemental composition 92 4.3.3 Amounts of TiO2 loaded on the buoyant composite photocatalysts 97 4.3.4 Light absorbance 99 4.3.5 Photocatalyst performance for MO dye degradation 100 4.3.6 MO dye degradation pathway 102 4.4 Conclusions 106 Chapter The Effect of Thickness of Photocatalyst Film Immobilized on the Buoyant Composite Photocatalysts on Their Property and Performance 108 5.1 Introduction 108 5.2 Experimental 111 5.2.1 Preparation of the buoyant composite photocatalysts with different film thicknesses 111 5.2.2 Photocatalyst characterization 112 5.2.3 Photocatalytic degradation experiments for MO dye in aqueous solutions 113 5.2.4 Modeling analysis of MO dye degradation kinetics 115 5.3 Results and discussion 117 5.3.1 TiO2 film thickness of the prepared buoyant composite photocatalyst 117 5.3.2 Effect of film thicknesses on MO dye degradation performance 118 5.3.3 Effects of the UV and Vis lights on the performance of the buoyant composite photocatalyst with different photocatalyst film thicknesses 122 5.3.4 Active photocatalyst film thickness under the UV and Vis light irradiations 125 5.4 Conclusions 130 Chapter A Preliminary Study of Buoyant Composite Photocatalysts Containing an Adsorbent Component and Their Performance in Phenol Removal from Aqueous III Solutions 132 6.1 Introduction 132 6.2 Experimental 135 6.2.1 Preparation of buoyant composite photocatalysts with an adsorbent component 135 6.2.2 Characterization and evaluation of the composite photocatalysts 136 6.2.3 Adsorption and photocatalytic regeneration experiments 137 6.3 Results and discussion 139 6.3.1 Morphologies of the new composite photocatalyst 139 6.3.2 Adsorption and degradation results of the composite material 140 6.3.3 Results in phenol removal performance from the two-stage adsorption and regeneration process 142 6.4 Conclusions 147 Chapter Conclusions and Suggested Future Studies 148 7.1 Conclusions 148 7.2 Suggested future studies 152 Reference 155 Publications 171 IV Summary Titanium dioxide (TiO2) has been extensively studied as one of the best choices of photocatalysts, attributing to its high activity and stability, non-toxicity and low cost The band gap of the most popular structure of TiO2, anatase, is around 3.2 eV The activation of TiO2 therefore needs light in the ultraviolet (UV) range with the wavelengths shorter than 388nm Thus, TiO2 photocatalyst is usually used under the UV irradiation However, the global energy crisis in recent years has urged the use of new and alternatively cheaper energy sources such as the sunlight It is logically more advantageous to be able to use the natural sunlight than the UV light from engineered lamps as the light source for photocatalytic reactions, especially in the environmental field application Since the solar light (another name of sunlight) that reaches the earth’s surface consists mainly of (about 45 %) the visible light (400 ~ 700 nm) but only a small fraction (around %) of the UV light (200 ~ 400 nm), the direct application of conventional TiO2 under the solar light radiation is therefore not effective for photocatalysis In addition, another fact is that both UV and visible lights attenuate quickly with the depth in water, as compared to that in air A possible solution to the problems mentioned above is to develop photocatalysts that can be photo-activated under the solar radiation, particularly under the visible light, and can be used at around the water-air interface In this thesis, TiO2 photocatalysts were modified and immobilized on a buoyant substrate (polypropylene) to obtain a buoyant composite photocatalyst that is effective to the visible light as well as the UV lights V and can be applied at water surface The developed buoyant composite photocatalyst was tested for the degradation of organic pollutants (dye and phenol) under various simulated light irradiation conditions Specifically, the work included the development of a low temperature hydrothermal method to immobilize modified TiO2 nano particles on the polypropylene (PP) substrate Then, an improvement in the TiO2 loading on the PP substrate was attempted and successfully achieved Following the preparation, the effect of thickness of the immobilized TiO2 film on the PP substrate on the photocatalytic reaction performance of methyl orange (MO) dye was examined Finally, the prepared buoyant composite photocatalyst was investigated in a two-stage adsorption and photocatalytic regeneration process with an adsorbent component for the performance in phenol removal from aqueous solutions In the first part, TiO2 was modified by doping mainly nitrogen and immobilized on PP granules (PPGs) to prepare a buoyant composite photocatalyst with visible light activities TiO2 nano sol was first prepared in the presence of acetyl acetone (AcAc) or acetic acid (AcOH) as the inhibiting agent and subsequently modified with triethylamine (TEA) A one-step low temperature (150 ºC) hydrothermal process was developed for the simultaneous crystallization and immobilization of the treated TiO2 nano particles on the PP substrate The difference of the inhibiting agents to TEA-modification and the effect of TEA treatment time on photocatalyst light absorption properties were investigated It was found that a longer treatment time of TEA on TiO2 sol enhanced the visible-light photoactivity and the inhibiting agent AcAc provided a better result for the TEA treatment than that of AcOH VI Characterization analysis with UV-Vis spectroscopy, Raman spectroscopy, X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy were conducted The crystal structures of the prepared TiO2 photocatalysts were found to be mainly anatase but a small amount of brookite The crystal size of the modified TiO2 photocatalyst was at about nm in the particles but around 30 nm in the film on PP substrate, attributed to the different nucleation mechanisms Both XPS and Raman spectra confirmed the existence of the nitrogen-doped composition (i.e, TiO2-xNx) but did not exclude the possibility of carbon-doped structure Degradation of MO dye with the prepared buoyant composite photocatalyst was examined and good degradation performance was achieved under both UV and visible lights In the second part, the focus was to increase the TiO2 loading that can be immobilized on the PP substrate to obtain a buoyant composite photocatalyst with a better photo-reactivity In stead of PP granules, polypropylene fabric (PPF) was used as an alternative substrate in the experiment A layered rutile and anatase TiO2 configuration was developed to achieve greater amounts of immobilization of TiO2 photocatalyst on the PPF The achieved high loading of TiO2 on the buoyant composite photocatalyst with this new immobilization configuration was attributed to the bottom rutile TiO2 layer that constituted from heaps of small flower-like structures on the PPF and thus provided a high specific surface area for the top anatase TiO2 layer to be immobilized The prepared buoyant composite photocatalyst with the rutile and anatase TiO2 configuration was found to be the most efficient one in MO dye VII degradation as compared to other configurations tested MO dye (15 mg/L) was completely degraded within h under the irradiation of a 150 W xenon lamp From the High Performance Liquid Chromatography analysis, it was found that the MO dye degradation possibly followed different degradation pathways under the UV or visible light irradiation More intermediate by-products were observed during the degradation process under the visible light than under the UV light and it took longer time for these intermediate by-products to be completely degraded under the visible light The results showed the importance of developing buoyant composite photocatalyst with high TiO2 photocatalyst loading, especially to improve the photoactivity under the visible light irradiation In the third part of the study, the logical research interest was directed to examine the effect of 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substrate”, presented at the 13th International Conference of TiO2 photocatalysis: fundamentals and applications, Town & Country Resort, San Diego, California, USA, September 22~25, 2008 171 .. .THE DEVELOPMENT AND STUDY OF TITANIUM DIOXIDE BASED BUOYANT COMPOSITE PHOTOCATALYSTS FOR IMPROVED APPLICATIONS IN PHOTOCATALYTIC DEGRADATION OF ORGANIC POLLUTANTS IN AQUEOUS SOLUTIONS. .. provide some preliminary information about the combination of the buoyant composite photocatalyst with an adsorbent component and the performance of the buoyant composite photocatalyst in phenol removal... (lower than the melting point of the plastic substrate) (b) To evaluate the photoactivity of the prepared buoyant composite photocatalysts for their performance in photocatalytic degradation of some