Effecient removal of various dyes from aqueous solutions by photocatalysis processes over modified titanium dioxide nanostructures

長庚大學化工與材料工程學系 博士論文 Department of Chemical and Materials Engineering Chang Gung University Doctoral Thesis 利用具奈米結構之改質二氧化鈦光催化程序以有效 去除水溶液中各種染料 Efficient Removal of Various Dyes from Aqueous Solutions by Photocatalysis Processes over Modified Titanium Dioxide Nanostructures 指導教授：莊瑞鑫 博士 Advisor: Ruey-Shin Juang, Ph.D 研究生：阮志秋 Graduate Student: Nguyen Chi Hieu 中華民國 109 年 01 月 January 2020 ACKNOWLEDGMENT I would like to thank all the people who have helped and supported me in many respects during the completion of this thesis First of all, I am heartily thankful to my supervisor Professor Ruey-Shin Juang, who is always ready to answer any question enthusiastically and give me helpful guidance He always encourages and supports me in both the academic research and ordinary life Hence, the research life in Taiwan becomes smooth and relaxing for me Without his guidance, this thesis would not have been completed I also would like to express my sincere gratitude to all the Professors at Chemical and Materials Department, Chang Gung University, who transmit me incomparable knowledge and great encouragement for my future career My thanks go to companions at Separation Materials and Techniques Laboratory and my friends at Chang Gung University, especially to Mr Chun-Chieh Fu for his time and assistance through the years of my doctoral program I also wish to thank Chang Gung University for providing me with a scholarship to study the Ph.D program Final support of this research work from the Ministry of Sciecne and Technology, Taiwan and the Chang Gung Medical Foundation, Taiwan is also gratefully appreciated My acknowledgment goes to my managers and colleagues at the Institute of Environmental Science, Engineering and Management, Industrial University of Ho Chi Minh City, Vietnam who facilitated me to pursue doctoral study Finally, I would like to dedicate my achievement and give special thanks to my beloved family, who always encourage, support, and believe in me It is a great comfort and relief to know that all my decisions are always supported and this inspires me to more My heartfelt thanks -iii- 摘要 利用光催化技術(Photocatalysis)進行有毒污染物之處理，為水及廢水淨化提供了一個 巨大的機會，以滿足不斷增長嚴重環境污染問題之處理需求。在眾多常見之非均相光觸 媒中，二氧化鈦(Titanium dioxide, TiO2)由於其高化學反應性和穩定性、無毒性和低成本， 已被證明是很有應用潛力之材料。為了進一步提高 TiO2 光觸媒對水中污染物之去除效率， 本論文嘗試以物理化學方式針對 TiO2 加以改質，以克服 TiO2 固有之缺點，並評估其應用 於廢水脫色(Decolorization)之潛力。通過添加貴金屬鉑(Platinum)及鈀(Palladium)、還原態 氧化石墨烯(Reduced graphene oxide, rGO)及與 ZnO 結合使用，製備出不同之 TiO2 基材光 觸媒，以改進材料之光催化活性(Photocatalytic activity)。本論文對各種材料首先鑑定其物 理化學性質，包括晶體結構(Crystal structure)、形態(Morphology)、表面積(Surface area)、 帶隙(Bandgap)等之變化。材料鑑定結果證明，影響光催化活性之關鍵參數有形態、表面 積、結晶度、活性面、帶隙及電子—空洞對(Electron-hole pairs)之壽命。此外，本論文通 過批次降解去除水溶液中各種染料包括亞甲基藍(Methylene blue)，若丹明 B (Rhodamine B)，甲基橙(Methyl orange)和萘酚藍黑(Naphthol blue black)之效率，以評估所製備材料對 染料之吸附能力及光催化活性，並與市售之 TiO2（Degussa P25）進行比較。降解實驗結 果表明，與自行製備之未改質 TiO2 相比，經各種改質 TiO2 之光催化活性有不同程度之提 升。此外，與 Degussa P25 相比，也有一些改質後材料具有更高之光催化活性，顯示其具 有未來實際應用之潛力 關鍵詞：去除染料；光催化程序；吸附；半導體；鈦酸鹽奈米材料；鈀金屬；鉑金屬； 還原態氧化石墨烯 -iv- ABSTRACT Using photocatalysis to apply for water purification and treatment has provided a great opportunity for the treatment of toxic contaminants in wastewater to meet the ever-increasing demands of handling serious environmental pollution problems Of many heterogeneous photocatalysts used, titanium dioxide (TiO2) is proven to be promising owing to its high chemical reactivity and stability, nontoxicity, and cost-effectiveness To improve the removal efficiency using TiO2 as an economically viable photocatalyst for wastewater treatment, the physicochemical modifications of TiO2 were adopted in this work Many different technical approaches were used to modify pure TiO2 to overcome the indigenous weakness of unmodified TiO2 This thesis study aimed to assess the potential employment of modified TiO2 photocatalyst for color removal of wastewater The different photocatalysts (TiO2-based materials) were prepared by various modified routes with the addition of noble metal (Pt, Pd), reduced graphene oxide (rGO), and combination with ZnO to improve the photoactivity of synthesized materials The changes in physicochemical properties (crystal structure, morphology, surface area, bandgap, etc.) of synthesized powders were studied Characterization experiments confirmed that the critical parameters such as morphology, surface area, crystallinity, active facets, bandgap and the lifetime of electron-hole pairs have the important roles for photocatalytic activity The photocatalytic activity of the synthesized materials was done in the liquid phase via the degradation of various dyes (methylene blue, rhodamine B, methyl orange, and naphthol blue black), in a laboratory-scale batch photoreactor The photocatalytic performance of all samples was compared with that of a commercially available TiO2 benchmark catalyst (Degussa P25) Photocatalytic performance tests revealed improved photocatalytic activity for the modified TiO2 compared to unmodified TiO2 prepared with the same method Also, several samples presented even higher photocatalytic activity compared to Degussa P25, demonstrating the promising potential for practical applications -v- Keywords: Dye removal; Photocatalysis; Adsorption; Semiconductors; Titanate nanomaterials; Palladium; Platinum; Reduced graphene oxide -vi- ABBREVIATION LIST UPLC®-MS/MS Ultra-Performance Liquid Chromatography-Tandem Mass Spectrometry TIP Titanium Isopropoxide XRD X-Ray Powder Diffraction XPS X-Ray Photoelectron Spectroscopy SEM Scanning Electron Microscope TEM Transmission Electron Microscope HR-TEM High-resolution Transmission Electron Microscope BET Brunauer Emmett Teller PL Photoluminescence Spectra FTIR Fourier transform infrared spectroscopy DRS UV- visible diffuse reflectance spectra TOC Total Organic Carbon IUPAC International Union of Pure and Applied Chemistry MB Methylene Blue MO Methyl Orange RhB Rhodamine B NBB Naphthol Blue Black -vii- TABLE OF CONTENTS Recommendation Letter from the Thesis Advisor Thesis/Dissertation Oral Defense Committee Certification ACKNOWLEDGMENT iii ABBREVIATION LIST vii TABLE OF CONTENTS viii LIST OF FIGURES xiii LIST OF TABLES xviii CHAPTER LITERATURE REVIEW 1.1 Photocatalyst 1.1.1 Physical properties and characteristics of titanium dioxide (TiO2) 1.1.2 The synthetic method for photocatalyst nanostructures 1.1.2.1 Sol−gel method 1.1.2.2 Hydrothermal/solvothermal method 1.1.2.3 Microwave method 1.1.3 The photocatalytic mechanism 1.1.4 Photocatalytic lifetime 11 1.1.5 Effect of operating parameters in photocatalysis 12 1.1.5.1 Effect of pH values 13 1.1.5.2 Effect of catalyst loading 14 1.1.5.3 Effect of the pollutants and their initial concentration 14 1.1.5.4 Light source 15 1.1.6 The limitation of photocatalysis in the practical systems 16 1.1.7 Modification method to enhance photocatalytic activity 17 1.2 Application of photocatalysts in dye removal 22 -viii- 1.3 The scope of this thesis 28 CHAPTER II MATERIALS AND METHODS 30 2.1 Materials 30 2.1.1 General chemicals 30 2.1.2 The characterization of contaminants 30 2.1.2.1 Methylene blue (MB) 31 2.1.2.2 Rhodamine B (RhB) 31 2.1.2.3 Methyl orange (MO) 31 2.1.2.4 Naphthol blue black (NBB) 31 2.2 Preparation procedure of materials 32 2.2.1 Preparation of GO sheets 32 2.2.2 Preparation of titanate nanomaterials (TNMs) 33 2.2.3 Preparation of TNMs-supported Pt nanoparticles 33 2.2.4 Preparation of reduced graphene oxide/titanate nanotube (rGO/TNT) composites 34 2.2.5 Preparation of Pd- TiO2 catalysts 34 2.2.6 Preparation of TiO2/ZnO/rGO composites (TZR) 35 2.3 The methods using to investigate the physical and chemical characterization of TiO2based nanostructures 36 2.3.1 X-ray powder diffraction (XRD) 36 2.3.2 Raman analysis 36 2.3.3 Electron microscope measurements 36 2.3.4 X-ray photoelectron spectroscope 37 2.3.5 Physical surface area and pore size measurements 37 2.3.6 Fourier transform infrared spectroscopy (FTIR) 37 2.3.7 UV-visible diffuse reflectance spectra (DRS) 38 2.3.8 Photoluminescence spectroscopy (PL) 38 -ix- 2.2.2 Preparation of titanate nanomaterials (TNMs) TNMs with different morphologies were prepared by a facile hydrothermal process for various reaction times as described previously (Chen et al 2013) In brief, 1.6 g of TiO2 (P25) was dispersed in 100 mL of 10 mol L-1 NaOH solution under vigorous stirring and sonication for 30 Afterward, the suspension was transferred into a Teflon-lined stainless-steel autoclave and kept in an oven at 130○C for and 24 h Pure TNS and TNT were obtained after 3- and 24-h hydrothermal processing, respectively After the reaction was completed, the autoclave was cooled to ambient temperature, and the solids were then filtered and washed by 0.1 mol L-1 HCl and distilled several times until the pH reached around 6.5-7.5 The resulting white powders were dried in a vacuum oven at 70○C for 24 h 2.2.3 Preparation of TNMs-supported Pt nanoparticles The TNS and TNT-supported Pt catalysts were prepared by photo-deposition as follows: 3.0 g of the pristine TNMs was introduced to 450 mL of 33.3 vol% ethanol solution; to this mixture, an appropriate volume of H2PtCl6 aqueous solution (0.1 mmol L-1) was then added to obtain a nominal Pt content with a weight percent of 0.5 They were illuminated under an ultraviolet lamp and stirred for h Light intensity on the pristine surface materials was approximately 6.5 mW cm2 for Pt photo-deposition The catalysts were subsequently separated by centrifugation and washed several times by distilled water The resulting powders were dried in an oven at 70○C for 24 h to remove moisture Photo-reduction of Pt+4 ions to platinum nanoparticles (Pt0) occurred and then highly dispersed Pt particles were deposited onto the TNS and TNT surfaces The as-prepared catalysts were denoted as Pt0.5-TNS and Pt0.5-TNT, corresponding to the sample containing 0.5 wt.% Pt -33- 2.2.4 Preparation of reduced graphene oxide/titanate nanotube (rGO/TNT) composites The rGO/TNT composite was hydrothermally synthesized as stated earlier (Wei et al 2016) but with some modifications In brief, 3.0 g of P25 and an amount of GO (1%, 2%, 3%, and 5% by weight) was added into 90 mL of NaOH (10 mol L-1) under vigorous agitation for 1.5 h and was sonicated for h to make it homogeneous The resulting solution was shifted into a stainless-steel autoclave (Teflon-lined), which was kept at 135oC in an oven for 24 h Afterward, the autoclave was cooled to 25oC The obtained solid was washed by 0.1 mol L-1 HCl until the pH of the washing solution became 1.5 The mixture was then agitated continuously for 24h to ensure ion exchange to occur The suspended solids were recovered by centrifugation at 10,000rpm and rinsed with DI water several times until the washing water was neutral The final products were dried at 80oC in a vacuum oven for 24h and calcined at 300oC for 1h in a tube furnace under Ar atmosphere to refine the crystal form The solid catalysts were denoted as 1%, 2%, 3%, and 5% rGO/TNT, respectively Pure TNTs were prepared by the same procedures as above but without the addition of GO 2.2.5 Preparation of Pd- TiO2 catalysts The Pd-TiO2 catalysts were synthesized by a facile sol-gel method as stated earlier (Messih et al 2017; Cheng et al 2018) with some modifications Briefly, 15 mL of TIP and varying amounts of Pd(NO3)2 with 0.25, 0.5, 0.75, and 1.0 wt.% of Pd in Pd-TiO2 was dissolved in 50 mL of isopropanol and agitated by magnetic stirrer for h to yield homogeneous solution Then, mL of wt.% of chitosan in acetic acid was added into this mixture and stirred continuously for h In the next step, mL of deionized water was drop-wised to the resulting solution until turbid sol was created Afterward, the obtained sol was left for 48 h for aging until gel particles appeared The suspension was filtered and washed several times with deionized water, and dried at 100oC in an oven overnight The sample was finally annealed at 500oC in a furnace for 3h to remove chitosan -34- and stabilize mesoporous and crystal structure of the materials Pure TiO2 was also synthesized by the same routes as above, except for the absence of Pd(NO3)2 2.2.6 Preparation of TiO2/ZnO/rGO composites (TZR) TZR were synthesized by a hydrothermal process utilizing graphene oxide (GO), zinc acetate, and titanium fluoride as the precursors First, GO was fabricated following the improved Hummers’ method, as stated above (Nguyen et al 2019) Subsequently, a suitable amount of GO (1%, 3%, 5%, 10% by weight) in 45 mL of ethanol was ultrasonicated for 30 to exfoliate GO completely (denoted as solution A) The appropriate zinc acetate and titanium fluoride (Ti/Zn molar ratio of 1∶1) were mixed in 45 mL of DI water, followed by vigorous stirring and sonicating for 15 in each process to obtain a homogeneous solution (solution B) After that, the solution A was added to solution B and stirred magnetically for h The ammonia water (NH4OH) was added dropwise into the suspension to adjust the pH to be 11 Then, the resulting suspension was shifted into a Teflon-lined autoclave under a controlled temperature of 180°C for 20 h Under hydrothermal conditions, ethanol in water acted as a reductant to convert GO into rGO Simultaneously, TiO2 and ZnO nanoparticles were also formed The precipitate was washed several times with DI water and ethanol to remove the impurities The final solid was dried in a vacuum oven overnight at 70°C and then calcined at 300°C for 30 They were denoted as TZR1, TZR3, TZR5, and TZR10 when 1, 3, 5, and 10 wt.% of GO was initially added, respectively For comparison, ZnO and TiO2/ZnO (TZ) were also prepared by the same procedures as above except in the absence of GO -35- 2.3 The methods using to investigate the physical and chemical characterization of TiO2based nanostructures 2.3.1 X-ray powder diffraction (XRD) XRD was analyzed to examine the crystal structure of the catalysts by a diffractometer (Bruker D2 PHASER, Karlsruhe, Germany) with CuKα radiation The results were recorded with a scan rate of 2/min over the angular range (2θ) from 10 to 80, the working voltage and current were 40 kV and 40 mA, respectively The crystalline phase of the nanoparticles was identified by comparing the major peak positions with standard JCPDS files The average crystallite size of as-prepared catalyst is estimated by the Scherrer equation given by D = kDS λ/β cosθ, where D is the crystal size, λ is the wavelength of X-ray radiation (0.15406 nm) for CuKα, kDS is taken as 0.89, and β is full width at half maximum (FWHM) of the peak intensity 2.3.2 Raman analysis Raman scattering (2D-CARS) spectroscopy, are also applied for characterizing the synthesized nanomaterial Herein, Raman spectra were collected by Raman analyzer (UniDRON model, UniNanoTech Co Ltd, Gyeonggi-do, Korea) 2.3.3 Electron microscope measurements The morphologies and surface characteristics of the synthesized catalysts were observed using Scanning electron microscope (SEM; SU8220 Hitachi, Tokyo, Japan) at an accelerating voltage of 10kV, transmission electron microscope (TEM; JEOL JEM 1230, Tokyo, Japan), and high resolution transmission electron microscope (HR-TEM, JEOL model JEM 2100 Plus, Tokyo, Japan) Before SEM measurement, the samples were coated with thin platinum Meanwhile, for TEM or HR-TEM measurements, the synthesized nanomaterials were dispersed in distilled water -36- and followed by the sonication to obtain the homogeneous solution, then this solution was dropped on the copper grid and dried in vacuum oven before analysis 2.3.4 X-ray photoelectron spectroscope The surface chemical composition of all samples was determined by XPS spectrometer (Fison VG ESCA210, Thermo Scientific, MA, USA) with Mg-Kα radiation source The binding energies were normalized to the signal for adventitious carbon at 285.0 eV The spectra were smoothed and a non-linear background was subtracted The element spectra were deconvoluted using a nonlinear least-squares fitting program with asymmetric Gaussian function 2.3.5 Physical surface area and pore size measurements Brunauer-Emmett-Teller (BET) specific surface areas and pore size measurements of the specimens were obtained utilizing the nitrogen adsorption-desorption apparatus (ASAP2020, Micromeritics, Norcross, GA, USA) Before analysis, all samples were degassed at 343 K for 12h The adsorption isotherms were used to depict the pore-size distribution in Barret-Joyner-Halender (BJH) way 2.3.6 Fourier transform infrared spectroscopy (FTIR) The bonding characteristics of functional groups in as-prepared materials were identified by FTIR spectroscopy Functional groups in the prepared samples were identified by a Fourier transform infrared spectroscope (FTIR; Bruker Tensor 27 IR, Karlsruhe, Germany) Prior to analysis, the samples were pressed pellets of a mixture of the sample powder with KBr The sample powder was diluted in KBr at a maximum ratio of 1%wt -37- 2.3.7 UV-visible diffuse reflectance spectra (DRS) UV-vis spectroscopy is a useful technique to determine the absorption properties of semiconductors Besides, the measurement of DRS allows estimating the band-gap energy value of semiconductor material This parameter is critical in the field of photocatalysis since it determines the light energy to be used to activate the semi-conducting solids (Abou-Gamra et al 2016) DRS spectra of prepared samples were measured at 25oC to study optical properties on a spectrophotometer (Jasco V650, Tokyo, Japan) equipped with an integration sphere (Accessory ISV-722, Japan) Bandgap energy value was estimated from corresponding Kubelka-Munk functions, F(R) = (1-R)2/R, which is proportional to the absorption of radiation, by plotting (F(R).hν)1/2vs.hν (López et al 2012) 2.3.8 Photoluminescence spectroscopy (PL) The PL spectrum is an effective method to investigate the electronic structure and the optical and photochemical properties of semiconductor materials, giving information on surface oxygen vacancies and defects and the efficiency of charge carrier trapping and transfer Herein, the photoluminescence (PL) spectra at room temperature of all samples were measured using a Spectrofluorometer (FP-8200, Jasco, Japan) 2.3.9 Photoelectrochemical characterization To confirm photo-response ability and photogenerated charge recombination rate in photocatalyst, transient photocurrent measurement is an efficient method The photocurrent values obtained by chronoamperometric measurements are used to determine the charge separation efficiency of photocatalysts These measurements are usually conducted in a three-electrode system consisting of working, reference, and counter electrodes and an electrolyte solution such as aqueous Na2SO4 and under UV, Vis, or UV Vis irradiation The measurements can be used to compare the current values at a constant voltage obtained in the dark and under irradiation by -38- switching the light on and off at certain time intervals as well as to compare these values of different photocatalysts at the same time (Yurdakal et al 2019) In this work, the transient photocurrent responses were measured on a model CHI 6081C (CH Instrument) work-station using a threeelectrode mode with a Pt electrode as the counter electrode, an Ag/AgCl electrode (saturated KCl) as the reference electrode, and the working electrode which was prepared as follows: the powders of as-prepared materials were pasted onto the conductive fluorine-doped SnO2 glass substrate (1.0 cm × 2.0 cm FTO) with an effective working area of 1.0 cm2 by the doctor blade method, and were calcinated at 450C for 2h The paste was made using acetylacetone (Sigma-Aldrich) and deionized water as a solvent with a few drops of Triton X-100 for an even deposition Photocurrents were measured in 0.5 mol L-1 of Na2SO4 solution with the above three-electrode system A 300-W xenon lamp was used as a light source (Nguyen et al 2018; Nguyen et al 2019) 2.4 Experimental procedure 2.4.1 Adsorption of dyes on the synthesized catalysts Batch adsorption tests were carried out to determine the amounts of dye adsorbed on asprepared materials All tests were set up in the dark to avoid the effects of photolysis and photocatalysis of dyes These experiments were carried out by adding 0.2 g of catalysts to 200 mL of dye solution in the concentration range of 5-185 mg L-1 under magnetic stirring for h at 25±1C The initial solution pH was kept constant at 7.0±0.2 Samples were taken at given time intervals (10 min) and the catalysts were separated from the solution by filtration through a 0.22-μm membrane (Chromophil® Xtra, Germany) The concentration of dye in the sample was measured by a UV-visible spectrophotometer (Jasco V650, Japan) at each suitable wavelength The amount of dye adsorbed on the catalyst after equilibrium, qe (mg g-1), was calculated by the following equation: (Gupta et al 2014; Asfaram et al 2015) qe  C0  Ce V (2-1) W -39- where C0 and Ce are the initial and equilibrium concentrations of dye in solution (mg L-1), respectively, V is the volume of solution (L), and W is the mass of the catalyst (g) Herein, the common two-parameter Langmuir and Freundlich equations were used to depict the equilibrium adsorption of dyes on as-prepared catalysts (Dashtian et al 2018) The linear forms of both equations can be represented by Eqs (2) and (3), respectively:   Ce Ce    qe qmax K L  qmax  (2-2) ln qe  ln K F  ln Ce n (2-3) where qmax is the maximum adsorption capacity corresponding to monolayer coverage (mg g-1) and KL is the Langmuir constant (L mg-1) in Eq (2-2) In Eq (2-3), KF is the Freundlich constant ((mg g-1)(mg L-1)n) and n is the constant representing adsorption intensity 2.4.2 Photocatalysis experiment The photocatalytic activity of as-prepared catalysts was evaluated on the degradation of dyes under UV/simulated solar light irradiation 2.4.2.1 With UV light irradiation Photocatalytic experiments under UV light irradiation were conducted in a laboratory-scale batch photoreactor as described in Fig 2-1 The reactor system included a cylindrical Pyrex-glass cell with a volume of 1.0 L and a 100-W high-pressure mercury lamp (HL100CH-5, Sen Lights Co., Osaka, Japan) placed in a 50-mm diameter quartz tube The UV intensity was 6.5 mW cm-2 measured by a UV radiometer (UV-MO03A, ORC Manufacturing Co., Tokyo, Japan) Both the lamp and tube were then immersed in the photoreactor with a light path of 80 mm An aliquot of the catalyst was placed in the reactor, to which 1.0 L of a solution containing dyes with initial concentration of 20 mg L-1 was added The initial solution pH was adjusted by 0.1 mol L-1 NaOH -40- or 0.1 mol L-1 HCl The temperature was maintained at 25±1C with a water-cooling jacket and the suspension was stirred at 300 rpm Prior to irradiation, the solution containing catalyst and dyes was magnetically stirred in dark for 40min to reach adsorption/desorption equilibrium The mixture was then subjected to UV irradiation from 60 to 180 Aqueous samples were taken at preset time intervals using a 0.22-μm syringe filter (Chromophil® Xtra H-PTFE-20/13, Germany) to remove suspended solids Experiments were duplicated under identical conditions The degradation efficiency of dyes was expressed in terms of decolorization and mineralization The reusability of photocatalyst was typically investigated for five repeated cycles After each cycle, the catalyst was collected by centrifuging, washed several times with DI water and ethanol Then, 30 mL of 30 vol% H2O2 was added into catalyst solution (3.0 g L-1), which was irradiated by UV for h and rinsed several times with DI water The catalysts were regenerated by drying and calcining in an oven at 300C for 1h before the beginning of the next cycle Fig 2-1 Schematic diagram of photoreactor using UV lamp -41- 2.4.2.2 With simulated solar light irradiation To study photocatalytic activity of the synthesized materials under simulated solar light, the suitable mass of photocatalysts was dispersed in 100 mL aqueous solution containing dyes (20 mg L-1) The mixture was then placed in a quartz cell and stirred magnetically for 40 in the dark to achieve absorption/desorption equilibrium The simulated solar source with a horizontal luminous intensity of 700 cd was created by a Xenon 300 W lamp solar simulator (model 6258, MKS Instruments, Inc., California, USA) with the emission spectrum of the lamp simulates solar radiation and The lamp was located at a distance of 15 cm above the top surface of the dye solution This batch photoreactor was described in Fig 2-1 The sequence later of the experiment were similar to the aforementioned part Fig 2-2 (a) Schematic diagram of photoreactor using solar simulator, (b) Spectral irradiance of simulated solar light (model 6258) -42- The kinetics of dye photodegradation over as-synthesized materials are analyzed by the common Langmuir-Hinshelwood (L-H) model (Mohammadzadeh et al 2015; Fu et al 2016), which is given as r  dCt kKCt  k  dt  KCt (2-4) where r is the reaction rate (mg L-1 min-1), t is the time (min), k is the rate constant (mg L-1 min-1), σ is the fractional coverage of catalyst surface, and K is the adsorption constant (L mg-1) When adsorption is relatively weak or the reactant concentration is low (KCt≪ 1), Eq (3) can be simplified to a pseudo-first-order kinetic model (Mohammadzadeh et al 2015; Fu et al 2016): dCt  kKCt  k appCt dt (2-5) where kapp is the apparent rate constant (min-1) Integrating Eq (4) gives C  ln t   k appt  C0  (2-6) The value of kapp is thus calculated from the slope of a plot of ln(C0/Ct) versus t 2.5 Analytical procedure 2.5.1 Monochromatic spectroscopy measurement of dyes The residual concentration of dyes was determined colorimetrically using a monochromatic spectroscopy method The correlation of dye concentration and its intense characteristics peak at the respective maximum wavelength using a UV-Vis spectrophotometer (Jasco V650, Tokyo, Japan) was developed The spectrophotometric absorbance of each individual dye solution (MB, RhB, MO, NBB) was measured at its corresponding maximum wavelength (λmax) of 665nm, 551nm, 464nm, and 618nm respectively by a UV-Vis spectrophotometer The unknown concentration of dyes in the solution was then calculated from calibration curves of adsorption versus concentration constructed from their standards -43- In the mixtures, the concentration of each dye was determined from a calibration curve of the concentration versus the first-order derivative of the absorbance by the considerable spectral overlap between two single dyes and their mixtures (Hoang et al 2014; Nguyen et al 2016; Dashtian et al 2018) The concentrations of RhB and MO in binary solutions were examined by first derivative spectrometry at 575 and 445 nm, respectively, whereas those of RhB and MB were obtained by the first derivative at 507 and 625 nm, respectively Fig 2-3 The first derivative spectra of (a) MB-RhB and (b) MO-RhB mixtures (The zero-crossing method shows the determined wavelengths of MB and RhB at 625 and 507 nm, respectively, in the MB-RhB mixture In the MO-RhB mixture, the wavelengths of MO and RhB were identified at 445 and 575 nm, respectively) The percentage color removal (decolorization) of the solution containing dyes were calculated from the concentration changes as below equation: decolorization (%) = 100 × C0 −Ct (2-7) C0 where C0 and Ct are the concentrations of dye in the solution at the beginning and time t, respectively -44- 2.5.2 Total organic carbon (TOC) TOC value is known as the total concentration of organics in the solution; therefore, in photocatalysis, the percentage of TOC removal can be used to reflect the mineralization of dyes TOC of dye solution was determined through the combustion of the sample at 750C using anondispersive IR source by a TOC Torch analyzer (Teledyne Tekmar Torch Combustion, Mason, OH, USA) The percentage TOC removal (mineralization) of the solution containing dyes were calculated from TOC value changes as below equation: mineralization (%) = 100 × [TOC]0 −[TOC]t [TOC]0 (2-8) where [TOC]0 and [TOC]t are the TOC values of dye solution at the beginning and the time t, respectively 2.5.3 The identification of the transformation products of the dyes during photocatalysis To identify the intermediates produced in photocatalysis of the dyes, samples were analyzed by tandem quadrupole mass spectroscopy Waters Xevo TQ-XS equipped with an ACQUITY UPLC® I-class system (Milford, MA, USA) An ACQUITY UPLC® BEH C18 column (2.1x50 mm, particle size 1.7 μm) was used for the separation of intermediates at 35C The flow rate of the mobile phase was in the range 0.15-0.35 mL min-1 For the analysis of MB and intermediates in photodegradation process, the mobile phase contained 0.1vol% formic acid (A) in deionized water and acetonitrile (B) whose program was as follow: 0.5 90% A, 0.5-2.8 90% B, 2.83.2 90-60% B, 3.2-3.5 60-10% B, and 3.5-5.0 10% B (Chen et al 2008; Kaur et al 2014) With the analysis of RhB and its intermediates, the mobile phase consisted of deionized water with 0.1vol% formic acid (A) and acetonitrile with 0.1vol% formic acid (B) The gradient elution was carried out as follows: 0.5 50-70% B, 0.5-3.0 70-95%B, 1.0 95% B, and 1.0 50% B The total time of the program was 5.0 minutes (Lu et al 2012) For the analysis of MO samples, the mobile phase included 0.01 mol L-1 ammonium acetate in deionized water (A) -45- and acetonitrile (B) (Chen et al 2008; Kaur et al 2014) The following gradient of mobile phase was adopted: 1.0 30-100% B, 1.0 100% B, 0.5 100-30% B, and 1.5 30% B The injection volume for all samples was μL The mass spectrometry was operated with electrospray ionization (ESI) in positive ion mode for MB and RhB and in negative ion mode for MO The capillary voltage ranged from 2.8 kV to 3.2 kV with a sampling cone voltage of 40 V and an extraction cone voltage of V The source and desolvation temperatures were at 90C and 350C, respectively The mass was scan in the range from 50-600 m/z The desolvation gas flow rate was 500 L h-1 Reaction intermediates were analyzed in triplicate The data were collected and processed with a MasslynxTM software (version 4.1) -46- CHAPTER TITANATE NANOMATERIALS–SUPPORTED PLATINUM NANOPARTICLES 3.1 Introduction In this chapter, the modification of TiO2 is carried out by the combination between the adjustment its morphologies and the introduction of the noble metals to improve photocatalytic activity due to enhance separation of electron-hole pairs and the synergistic effect of high adsorption ability and good photocatalytic activity (Chen et al 2013; Yi et al 2017; Yi et al 2018) It is well-known that adsorption is applied to enrich dyes from a large volume of the water body and the adsorbed dyes are then removed by photocatalysis Consequently, the combination of adsorption and photocatalysis processes has been shown to enhance pollutant removal The effects of dye structures and titanate nanomaterials (TNMs) morphologies on photocatalytic removal of dyes have been explored in this chapter Hence, four model dyes with different structures and charges were used for evaluating photocatalytic activity of TNMs, before and after Pt deposition, for single and binary dye solutions 3.2 Results and discussion 3.2.1 As-prepared catalyst characterization Fig 3-1 exhibits the XRD patterns of P25 and as-prepared materials The XRD pattern of P25 can be indexed to anatase-phase TiO2 (JCPDS No 89-4921) and rutile-phase TiO2 (JCPDS No 89-0552) (Lu et al 2015) The characteristic peaks in the XRD pattern of P25 were clear, strong, symmetric, and sharp, implying that TiO2 was mostly crystalline It was observed that the XRD patterns of TNS and TNT only presented weak diffraction peaks at 2θ = 25.3° (101), 37.8○ (004), and 48○ (200), corresponding to TiO2 anatase phase Meanwhile, the small diffraction peak of rutile phase at 27.5○ (110) could only be seen from the XRD pattern of TNS In contrast to XRD pattern -47- ... the best results in the case of solutions of phenol were obtained at pH 6.5 with 60.55% of removal, followed by 54.01%, 48.05%, and 39.95% of removal for solutions of phenol at pH 9, 3, and 7.2,... applied to enrich dyes from a large volume of water and the adsorbed dyes are then removed by photocatalysis Consequently, the combination of adsorption and photocatalysis processes has been... 0.016 to 0.027 min−1 and from 0.012 to 0.023 min−1, in case of NBB by photocatalysis using a polyaniline-coated titanium dioxide nanocomposite 1.1.6 The limitation of photocatalysis in the practical