Nghiên cứu tổng hợp vật liệu ceo2 tio2 nano ống và hoạt tính xúc tác phân hủy quang hóa trong vùng khả kiến tt tiếng anh

HUE UNIVERSITY HUE UNIVERSITY OF SCIENCES LE THI THANH TUYEN A RESEARCH OF CeO2/TiO2 NANOTUBES PREPARATION AND THEIR PHOTOCATALYTIC DEGRADATION UNDER VISIBLE LIGHT IRRADIATION Major: Theoretical Chemistry and Physical Chemistry Code: 62.44.01.19 PhD DISSERTATION ABSTRACT HUE, 2019 The thesis has been completed at Department of Chemistry, Hue University of Sciences, Hue University Supervisors: Prof Dr Tran Thai Hoa Dr Truong Quy Tung Examiner : Examiner : Examiner : : The dissertation will be defended at Time: date month year 2019 The dissertation could be found at: INTRODUCTION Being one of rare-earth metal oxides, CeO has attracted a great deal of attention due to its special electron orbital structure, the unique optical, Ce3+/Ce4+ redox behavior, high thermal stability and large oxygen-storage capability Surface defects such as oxygen vacancies working as electron traps can impede e –/h+ recombination and the 4f electron configuration can enhance the electron transfer process from the adsorbed dye to oxygen species The oxygen storage capacity makes Ce suitable to many applications as an important component of an automatic three-dimensional catalyst or an oxidation catalyst CeO2 is also used in many sensors, in fuel-cell technology as a solid-state electrolyte, and even in comestic chemistry The abilities to store and liberate oxygen in Ce are seemingly facilitated by the structure being similar to that of fluorite The oxygen in the crystals lies in parallel planes, allowing the oxygen atoms to diffuse effectively in order to form a network of oxygen-empty holes This is favourable for the oxidation of the solid Therefore, CeO2 has special properties in the transfer of electrons and the rise of absorb-light ability TiO2 is classified as a semiconductor widely used in photochemical techniques to decompose numerous kinds of toxic organic contaminants because of its outstanding features TiO is a low-cost non-toxic compound with high chemical durability, high photochemical stability and biological inertness However, its photochemical activity only activates under UV light irradiation due to its wide band gap (3.2 eV for anatase) and fast recombination of the photo-generated electron/hole pairs (10 –9 to 10–12 s) Thus, various approaches have been made to further improve the photo-catalytic performance of TiO2 including innovating physical properties of TiO2 like morphology, dimension and crystallite phase or doping/coupling TiO2 with other metallic elements or oxides In comparison with nanoparticles, TiO2–NTs possess photo-catalytic features Depending on the synthesizing method utilized, those preeminent features are massive surface (up to 478 m2/g), great volume of capillary (up to 1,25 cm3/g), capacity of transferring electrons from long distances, capacity of ion exchange, and noticeable capacity of absorbing light as a result of the considerable proportion between the length and the diameter of the tube The combination of TiO2 with CeO2 is expected to significantly improve the catalytic activity that derives from the role of TiO2 as a mechanical, thermal and chemical stabilizer with the good dispersion of CeO2 nanoparticles The adjoining surface of this oxide system is referred as the unique center (Sui generis) which exhibits unique chemical properties Thus, TiO is becoming an important supporting substance which is investigated to elucidate the relationship between the structure and catalytic activity of the oxidemixture system Although the structure and properties of the CeO 2/TiO2 system have attracted strongly the development of the scientific research projects in recent years, scientific research projects on this material in Vietnam is quite limited There is no project to adequately study the structural characteristics as well as the applications in photochemical catalysis that relates to the published CeO 2/TiO2-NTs In view of the needs and current status of research in the country and in the world as a whole, as well as the research conditions in Vietnam, we chose a scientific research project titled: “A study of the synthesis CeO2/TiO2 nanotubes and its photocatalytic activity in the visible spectrum” NEW CONTRIBUTIONS OF THESIS This is the first time in Vietnam, we have been studied systematically about the synthesis of the CeO2-doped TiO2 nanotubes (CeO2/TiO2-NTs) and their visible light photocatalytic degradation behavior This is the first time, the mechanism of free radical formation of MB photocatalytic degradation over CeO 2/TiO2-NTs has been studied by using the fluorescence technique with terephthalic acid as probe and using tert-butanol as a hydroxyl radical scavenger This is the first time using the Arrhenuis and Eyring equations to study the photocatalytic degradation of MB by CeO2/TiO2-NTs in visible light The results showed that the photocatalytic degradation of MB by CeO 2/TiO2-NTs is controlled by diffusion and free hydroxyl radical reaction This is the first time the Box Behnken design of the response surface methodology was employed to optimize synthesis conditions for the photocatalytic degradation of MB over the synthesized CeO2/TiO2-NTs, including four experimental parameters namely; hydrothermal temperature, hydrothermal time, CeO 2/TiO2 molar ratio and calcination temperature Chapter LITERATURE REVIEW 1.1 Overview of photocatalytic reaction 1.2 Overview of TiO2 1.3 CeO2-doped TiO2 nanotubes (CeO2/TiO2-NTs) 1.4 Overview of response surface methodology (RSM) in optimization Chapter AIMS, CONTENTS AND EXPERIMENTAL METHODS 2.1 Aims Synthesis of TiO2 nanotubes and CeO2/TiO2 nanotubes for enhancing the photocatalytic degradation in the visible light 2.2 Contents 2.2.1 Synthesis of TiO2 nanotubes 2.2.2 Synthesis of CeO2-doped TiO2 nanotubes (CeO2/TiO2-NTs) 2.2.3 Study on the application of CeO2/TiO2-NTs in visible light photocatalytic degradation of dyes 2.3 Research methods 2.4 Experimental Chapter RESULTS AND DISCUSSION 3.1 SYNTHESIS OF CeO2/TiO2 NANOTUBES (CeO2/TiO2-NTs) 3.1.1 Synthesis of TiO2 nanotubes (TiO2-NTs) 3.1.1.1 Effects of hydrothermal temperature From the SEM image (Figure 3.1), it can be seen that the obtained TiO2 is shaped like an singe fiber of nanoparticles with the average size of about 240 nm at 140 °C Simultaneously, there is the accumulation of many rough surfaces on the fibers When rising to 160 °C, the tube structure of TiO2 appears more prominently than with the average size of 290 nm The tube structure is not only unclear but also appears the aggregation of several rods at 180 °C Additionally, thin nanoplates and nanoparticles form large and long bars At 200 °C, TiO2 likes thin, long and similar rods, which cannot see the agglomeration of nanoparticles, nanorods and nanoplates as at 180 °C 140 °C 160 °C 180 °C 200 °C Figure 3.1 SEM images of TiO2 synthesized hydrothermally at different temperature Table 3.1 Textural Properties of TiO2-NTs at different temperature Tempe SBET rature ( m /g) 140 °C Vpore dpore Adsorpti Hysteresis (cm /g) (nm) on type type 282 1,29 17,56 IV H3 160 °C 247 1,04 16,84 IV H3 180 °C 43 0,14 14,73 IV H3 200 °C 16 0,06 18,53 IV H3 The Table 3.1 shows that the increase in hydrothermal temperature leads to the decrease in the specific surface area and the volume of capillary as well as the change in capillary diameters When the hydrothermal temperature was 140 °C and 160 °C, the TiO2-NTs had a very large surface area, much larger than the P25 (50 m2/g) At 180 °C, the SBET of the TiO2-NTs declines by nearly six times, compared to that of hydrothermals at 160 °C Meanwhile, samples of the hydrothermal at 200 °C had S BET less than 16 times that of 160 °C More importantly, at over 160 °C, the S BET of the material is smaller than that of P25 3.1.1.2 Effects of hydrothermal time From the Figure 3.2, it can be seen that the main phase composition of the obtained TiO2 samples is in amorphous form along with the presence of Ti 9O17 and Na2Ti3O7 crystals in low diffraction intensity, which shows the poor crystallinity The presence of Na2Ti3O7 in hydrothermally synthesized TiO2 nanotubes has been reported in numerous published studies Anatase and rutile crystals appear in all samples at various temperatures, but the diffraction intensity is very weak The XRD result illustrates that the hydrothermal time does not significantly affect the composition, the crystal structure and the crystallinity of the composite material There was a sharp increase in the surface area of SBET when the hydrothermal time increased from 18 hours to 22 hours (from 107 m2/g to 275 m2/g), and the surface area decreased almost a half as the hydrothermal time increased to 24 hours Figure 3.2 XRD patterns of TiO2 synthesized at 160 oC for different hydrothermal time 3.1.2 Synthesis of CeO2/TiO2-NTs Figure 3.3 XRD patterns of TiO2-NTs; TiO2-NTs 550; CeO2/TiO2NTs@0.1 The XRD patterns of the TiO2–NTs, TiO2–NTs 550 (TiO2– NTs calcinated at 550 °C) and CeO 2/TiO2–NTs@0.1 are shown in Figure 3.3 It can be seen clearly that TiO 2–NTs before calcination mainly consists of amorphous phases with low diffraction intensity, showing poor crystallinity However, after being calcined at 550 °C, a sharp diffraction peak at 25.3° corresponding to the characteristic anatase (101) crystal plane appeared in the TiO 2–NTs 550 sample with a more noticeable intensity compared to that of the CeO doped TiO2 sample Diffraction peaks corresponding to the (004), (200), (105), (211), and (204) planes were also present in the XRD pattern In addition, a quite unclear diffraction peak near the position of 43° which can be assigned to the rutile (210) crystal plane was detected, revealing the remaining of the rutile phase in TiO 2–NTs This fact might serve as an evidence of the transformation of rutile to anatase during calcination Therefore, it is undeniable that, after being calcined at high temperature, there is an improvement in the anatase crystallinity and the anatase to rutile phase transformation The face centered cubic CeO found in the synthesized CeO2/TiO2–NTs sample is based on the presence of (101) and (200) characteristic peaks The existence of the anatase phase in CeO2/TiO2–NTs at the same position as in TiO 2–NTs 550 and the face centered cubic CeO2 phase is a firm proof for the separation of these oxides during the synthesis process; in other words, the synthesized materials exist as CeO2–TiO2 composites As a result, ceria could disperse mainly on the surface of the TiO nanotubes in the form of grains creating the boundaries in the synthesized composites (Figure 3.4b) Figure 3.4 TEM images of (a) TiO2–NTs, (b) CeO2/TiO2–NTs, (c) HRTEM image of CeO2/TiO2–NTs and (d) EDX spectrum of CeO2//TiO2–NTs Figure 3.5 Nitrogen adsorption/desorption isotherms of TiO2–NTs, TiO2–NTs 550 and CeO2/TiO2-NTs@0,1 3.1.2.1 Effects of calcination temperature It can be seen from the XRD spectra the calcination temperature had a strong effect on crystal structure and phase components of the catalysts The introduction of CeO in TiO2-NTs under proper calcination temperature (below 600 °C) did not change the nanotubes structure of starting material At 550 °C, the crystallization of the anatase phase obtained was the most perfect with the sharpest diffraction peak at 25.3° The CeO crystal structure found in the synthesized products at this temperature was also more complete with the clear characteristic peak at 28.5°.The surface area SBET decreased as the calcination temperature increased, especially when the temperature changed from 400 °C to 550 °C led to the decrease in SBET from 149 m2/g to 65 m2/g 3.1.2.2 Effects of doped ratio CeO2/TiO2 Compared to bare TiO nanotubes and bare CeO2, CeO2/TiO2-NTs@X clearly exhibits a broader absorption in the visible region (wavelength = 400-600 nm), displays a slight red shift 10 in the optical adsorption (Figure 3.6) The band gap values calculated of CeO2/TiO2-NTs is found to decrease with the increased doped ratio CeO2/TiO2 2.64 eV, lower than that of TiO2-NTs (3.08 eV) and CeO2 (2.93 eV) This red shift of the adsorption edge indicates the enhanced ability of the CeO2/TiO2-NTs hybrid catalyst to adsorb visible light The red shift in the optical transition as well as the decreased band gap value (Eg) of the CeO2/TiO2-NTs composite compared to TiO2-NTs and CeO2 were suggested to be relative to the presence of Ce3+ ions with one electron in the strong localized 4f orbitals as observed from the XPS spectra Figure 3.6 a) UV–Vis diffuse reflectance spectra and b) Tauc’ plots 3.2 THE PHOTOCATALYTIC ACTIVITY OF CeO2/TiO2-NTs 3.2.1 The MB adsorption of CeO2/TiO2-NTs 3.2.1.1 The point of zero charge (pHPZC) of CeO2/TiO2-NTs Δ pH 0.5 f(x) = 0.02x^2 - 0.23x + 0.65 R² = 0.99 0 10 12 -0.5 pH Figure 3.7 the pHPZC estimated using pH drift method of CeO2/TiO2NTs 11 The point of zero charge (pHPZC) of CeO2/TiO2-NTs estimated by the pH drift method is approximate 3.97 At pH < 3.97, the surface of the CeO2/TiO2-NTs is charged positively due to protonation and is charged negatively when pH > 3.97 3.2.1.2 The adsorption kinetics of MB on CeO2/TiO2-NTs The experimental data of the first order kinetic model with the high coefficient of determination (R2 = 0.912-0.963) implies that the adsorption process of MB on CeO2/TiO2-NTs followed the pseudo- first order kinetic model This means that the adsorption process was controlled by a physical adsorption and the rate-limiting step in this case involved a diffusion of MB to the surface of TiO nanotubes 3.2.1.3 The isotherm adsorption of MB on CeO2/TiO2-NTs The experimental data are analyzed according to the linear form of Langmuir and Freudlich model The both R2 values are high and significant p-value < 0.05 which indicates the equilibrium data are fixed well both isotherm model of Langmuir and Freundlich These results implies a monolayer adsorption and the existence of heterogeneous surface in the adsorbents, respectively 3.2.2 Photocatalytic degradation of MB over CeO 2/TiO2-NTs in the visible light 3.2.2.1 Photocatalytic behaviors of CeO2/TiO2-NTs in visible light Adsorption kinetics and photo-catalytic kinetics over several catalysts (TiO2–NTs 550, CeO2, CeO2/TiO2–NTs, and commercial P25 for sake of comparison) are represented in Figure 3.8 Practically, no change in MB concentration was noticed after light illumination for 150 minutes without any catalyst (blank sample) 12 This means that self-photolysis of MB was negligible Practically, the same result was found with CeO2 Adsorption of MB in the dark yielded around 15 %, 90 %, and 70 % on P25, TiO2–NTs 550, and CeO2/TiO2–NTs, respectively The TiO2–NTs 550 material exhibited strong adsorption of MB without showing any catalytic activity Whereas, although yielding around 70 % as ordinary adsorption, CeO 2/TiO2–NTs exhibited excellent photo-catalytic activity compared to P25 The MB photodegradation efficiency was 84.6 % over CeO2/TiO2–NTs after 60 minutes of light illumination and practically 100 % after 120 minutes, while P25 produced an efficiency only 46.7 % after 60 minute of illumination CeO2 showed the light adsorption, and afterward no photocatalytic degradation These results indicate that the composite of TiO2 and CeO2 has successfully improved the photo-degradation capability of TiO2 nanotubes The UV-Vis spectra for the photocatalytic degradation of MB over CeO-/TiO2-NTs shows that the maximum absorption at 664 nm (electron transfer –* in MB structure) decrease with an increase in light illumination time To confirm the mineralization of MB over CeO2/TiO2-NTs catalyst, the COD of reaction products were carried out The initial COD was 28.2 mg·L–1, and its decrease became faster as the illumination time increased reaching 10.68 mg·L –1 after 120 minutes These results confirmed the effectiveness of CeO 2/TiO2– NTs as a photo-catalyst for MB degradation under visible light After 120 minutes of illumination, the total decolorization of MB practically occurred, while around 61.1 % of COD reduction was obtained The difference between the degradation and mineralization could be attributed to the existence of intermediate products 13 Figure 3.8 The concentration changes of MB solution during visible light illumination over TiO2-NTs, P25, CeO2, CeO2/TiO2-NTs and Blank 3.2.2.2 Effects of different parameters on photodegradation of MB a Effects of initial MB concentration The results showed that the photodegradation of MB decreased from 97.61% to 47.54 when the MB initial concentration increased from 10 ppm to 30 ppm b Effects of solution pH It is found that the MB degradation efficiency increased sharply when pH was increased from to 4, then continued to increase slowly when the pH changed from to but then decreased sharply when the pH increased to 12 The point of zero charge (pHPZC) of CeO2/TiO2-NTs estimated by the pH drift method is approximate At pH < 4, MB molecule is neutral (pKa = 3.8) and the surface of the CeO2/TiO2-NTs is charged positively due to protonation Therefore, the Van der Waals interaction between the MB and CeO2/TiO2-NTs could be dominant Such poor interaction results in photochemical degradation reaction with very low efficiency Furthermore, with the positively charged surface of the 14 surface, it is possible to limit the hydroxyl ion supply required for free radical formation which is important for color decomposition MB degradation increases with increasing pH because the increasing electrostatic interaction between the negatively charged surface and positive cationic dye cause stronger photocatalytic reaction MB degradation peaks at pH = The higher the rate of degradation reaction at higher pH, the higher the amount of hydroxyl ion at the surface of the CeO2/TiO2-NTs which is the source forming hydroxyl radicals in the following equation: h+ + OH- → •OH However, the photochemical degradation is inhibited when the pH is too high because the hydroxyl ion competes with the MB molecules in adsorption on the photocatalytic surface The highest MB degradation efficiency is 99.66 at pH = c Effects of calcination temperature The results indicated that CeO2/TiO2-NTs@0,1 annealed at 550 °C show the best photocatalytic activity, and the degradation degree of MB under visible light was 97% during 120 minutes of irradiation However, a further increase of temperature to 600 °C resulted in a decrease in the photodegradation efficiency d Effect of doped ratio CeO2/TiO2 The photodegradation efficiency initially increased with an increase in doped ratio CeO2/TiO2 and the highest MB degradation efficiency was obtained if CeO2/TiO2 ratio of 0.1 However, the photodegradation efficiency decreased with further increase of Ce 3.2.2.3 The mechanism of free radical formation The free radical generation by photo-induced electron-hole pairs were confirmed by the fluorescence emission spectrum Figure 15 3.9a shows the induction of fluorescence from 5·10 −4 M terephthalic acid solution in 2·10−3M NaOH The increase in fluorescence intensity against illumination time at 425 nm was observed The fluorescence intensity by UV light illumination in terephthalic acid solutions increased almost linearly against time Consequently, we can conclude that OH radicals formed at the CeO 2/TiO2–NTs interface are in proportional to the light illumination In order to confirm the formation of hydroxyl radicals on the catalytic surface with visible light radiation the tert-butanol is used as the free radical scavenger (Figure 3.9b) In the presence of tertbutanol, a marked reduction in the MB photo-chemical catalytic activity was observed, and the decomposition efficiency decreased sharply when increasing the amount of tert-butanol Specifically, when adding 0.1 mL of tert-butanol, after 120 minutes of light illumination, the degradation efficiency of MB decreases by more than 30 % compared to the absence of tert-butanol (64.6 % vs 97 %) The presence of 0.2 mL tert-butanol decays to 58.3 % It is clear that the reaction is constrained by the presence of free radical scavenger The occurrence is related to the mechanism of free radical formation Figure 3.9 a) Fluorescence spectra observed during illumination of CeO2/TiO2–NTs in 2×10−3 M NaOH solution of 5.10–4 terephthalic acid; b) Influence of tert-butanol on the degradation of MB 16 Band edge energy at the interface of the n-semiconductor was calculated by means of equations proposed by Xu and Schoonen The ECB and EVB calculated are -0.23 and 2.85 eV for TiO 2, respectively and -0.405 and 2.525 eV for CeO 2, respectively The mechanism of MB photocatalytic degradation over CeO2/TiO2-NTs in the visible light can be proposed at Figure 3.10 N H E Figure 3.10 The proposed mechanism of MB photocatalytic degradation over CeO2/TiO2-NTs in the visible light 3.2.2.4 Effects of temperature – Transition state theory The effect of temperature on the rate of photodegradation was performed by varying temperature from 25 to 65 °C with constant initial MB concentration, catalyst loading and illimination time was 15 ppm, 0.08 g and 120 min, respectively Table 3.2 The values of rate constant under different temperature Temperature (°C) 25 35 45 55 65 k (min-1) R2 p 0,011 0,013 0,017 0,022 0,03 0,968 0,988 0,990 0,934 0,977 < 0,001 < 0,001 < 0,001 < 0,001 < 0,001 The MB photodegradation rate was found to increase quickly with increasing temperature Particularly, the photodegradation rate 17 increased more than 63% with the increase of temperature from 25 to 65 °C The activation energy Ea calculated using Arrhenius equation of MB photodegradation over CeO2/TiO2-NTs was 21.12 kJ.mol-1 Low activation energy (below 42 kJ.mol -1) implies diffusion controlled process The value of H # S # of MB photodegradation calculated from Eyring plot was illustrated in Table 3.3 Table 3.3 Activation parameters of of MB photodegradation Arrhenuis Temperature Ea R2 (kJ.mol-1) Transition state theory H # S # 21,12 Ea(t) R2 (kJ.mol-1) (kJ.mol- (kJ.mol- (kJ.mol-1) 25 35 45 55 65 G # 0,982 18,484 K-1) -0,221 84,313 86,522 88,731 90,940 93,149 ) 20,962 21,045 21,128 21,212 21,295 0,978 3.2.2.5 Optimization of synthesis conditions for photocatalytic degradation of MB by CeO2-doped TiO2 According to primary experiments, four factors including the hydrothermal temperature (X1), calcination temperature (X2), hydrothermal time (X3) and doped ratio (CeO2/TiO2) (X4) were considered to effect on the catalytic properties of CeO 2/TiO2-NTs In present study, the experimental design of Box–Behnken was employed to determine the optimum levels of the significant variables The number of experiments (N) required for the development of this design is defined as N = 2k(k-1) + C0, where k is the factor number and C0 is the replicate number of the central point 18 Thus a total of 27 runs performed for optimizing these four variables in the current Box– Behnken design High level New High D Cur 0.00000 Low Optimizati on Low level conditions X1 180.0 [163.2787] 140.0 X2 600.0 [557.2860] 500.0 X3 22.0 [19.6721] 18.0 X4 0.50 [0.10] 0.10 Composite Desirability 0.00000 Yield at optimizatio n conditions Y (%) Targ: 75.0 y = 0.9293 d = 0.00000 Figure 3.11 Optimization plot for the photo-catalytic decolourization yield of MB over CeO2/TiO2–NTs The profile for predicted values in the MINITAB–16 is employed for the optimization process The optimization design matrix (Figure 1) represents the maximum photo-catalytic decolorization (92.9 % for MB) at conditions set as: hydrothermal temperature (163 °C), calcination temperature (557 °C) hydrothermal time (20 h) and doped ratio (0.1 mol·mol –1) The reliability of this prediction was examined by performance of five similar experiment at optimization conditions The experimental decolorization yield for were 93 %; 96 %; 94.5 %; 95 % and 94.2 % The one-sample t-test show non-significant difference with respective value presented by model (t (4) = –2.32, p = 0.08) Therefore, the synthetic conditions were used to synthesize the CeO2/TiO2–NTs for further experiments 3.2.2.6 Photocatalytic kinetics of MB degradation Kinetics of adsorption and photo-catalytic degradation of MB over CeO2/TiO2–NTs is shown by Figure 3.12 19 Figure 3.12 Kinetics of adsorption and photo-catalytic degradation of MB over CeO2/TiO2–NTs at different initial MB concentrations Table 3.4 Apparent first-order rate constant values for the different initial concentrations of MB C0 (ppm) 10 15 20 25 30 35 kapp (min–1) 0.036 0.034 0.032 0.014 0.007 0.005 0.004 A linear plot of r0 0.18 0.34 0.48 0.28 0.15 0.15 0.13 ln (C0 a C ) R2 0.989 0.979 0.970 0.967 0.911 0.958 0.956 p < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 vs t yields the kapp The high coefficient of determination, R (0.911–0.989) confirms that the MB photo-catalytic degradation fitted well with the L–H first-order kinetic model Furthermore, as shown in Table 3.4, the apparent firstorder rate constants kapp declined with the increase of initial MB concentrations This might be resulted from the generated intermediate products during the photo-catalytic reaction; more particularly, the MB photo-degradation productivity is poorer since a 20 great amount of intermediates is adsorbed on the surface of CeO2/TiO2–NTs, which slows down the overall reaction rate The plot of kapp versus C0a shows a linear variation with high coefficient of determination (R2 = 0.969, p = 6.10–5) The values of kr and KL calculated from the intercept and the slope of straight lines for the photo-catalytic process were 0.103 mg·L –1·min–1 and 0.840 L·mg–1 respectively The determined adsorption equilibrium constant KL in our kinetic model is quite close to that estimated from the Langmuir adsorption isotherm, which also supports the validity of the Langmuir-Hinshelwood model for the photo-degradation of MB CONCLUSION The TiO2 nanotubes (TiO2-NTs) were successfully prepared via the simple and inexpensive hydrothermal route, using commercial anatase TiO2 powder as a precursor The results show that the obtained TiO2 by the hydrothermal treatment with 10 M NaOH at 160 °C for 20 hours possess uniform nanotubes with small outer diameters of about 10 nm, and 270 nm in length, and high specific surface areas of 246.65 m2/g CeO2/TiO2 nanotubes (CeO2/TiO2-NTs) were successfully synthesized by the impregnation of CeO2 on hydrothermally synthesized TiO2-NTs The effects of cerium doping content and calcination temperature on structure, morphology, composition, and visible light absorption property of CeO2/TiO2-NTs were also studied The results show that CeO 2/TiO2–NTs basically maintained the TiO2 the anatase crystal structure and a mixture of Ce 4+/Ce3+ oxidation states exists on the surface of the synthesized CeO 2/TiO2– NTs catalyst Compared to bare TiO nanotubes and bare CeO2, 21 CeO2/TiO2–NTs clearly exhibit broader absorption in the visible region, displaying a slight red shift in the optical adsorption This red shift of the adsorption edge indicates the enhanced ability of the CeO2/TiO2–NTs hybrid catalyst to adsorb visible light CeO 2-doped TiO2 nanotube is stable and potential as a visible-light photo-catalyst for organic substances degradation in aqueous solutions The photo-catalytic degradation of dyes over CeO2/TiO2-NTs in the visible region was systematically investigated The enhanced photo-catalytic activity of the hybrid catalysts involving CeO and TiO2 could be attributed to the synergistic effects between two oxides due to the presence of hetero-junctions and the Ce 3+ species in composites The fluorescence technique with terephthalic acid as probe and using tert-butanol as a hydroxyl radical scavenger demonstrate that the reaction was constrained by the presence of free radical scavenger The occurrence is related to the mechanism of free radical formation The more •OH radicals form at the CeO 2/TiO2– NTs interface, the higher MB photodegradation efficiency is This is the first time the mechanism of free radical formation of MB photocatalytic degradation over CeO2/TiO2-NTs has been studied by using the fluorescence technique with terephthalic acid as probe and using tert-butanol as a hydroxyl radical scavenger The equilibrium data is well-fixed both isotherm model of Langmuir and Freundlich The determined adsorption equilibrium constant KL in our kinetic model is quite close to that estimated from the Langmuir adsorption isotherm, which also supports the validity of the Langmuir-Hinshelwood model for the photo-degradation of MB The Arrhenuis and Eyring equations were used to obtain the activation thermodynamic parameters of MB adsorption such as 22 # activation enthalpy H , activation entropy S and activation free # energy G This is the first time the Arrhenuis and Eyring # equations were used to study the photocatalytic degradation of MB by CeO2/TiO2-NTs in visible light The results show that the photocatalytic degradation of MB by CeO2/TiO2-NTs was controlled by diffusion and free hydroxyl radical reaction The effective variables on the preparation of CeO 2/TiO2–NTs catalysts for photo-catalytic performance were optimized utilizing the Box–Behnken design (BBD) of response surface methodology (RSM) to find out the optimum conditions for obtaining the maximum photo-catalytic yield and the ability of obtained a catalyst to photo-degrade methylene blue (MB) under visible light This is the first time the Box Behnken design of the response surface methodology was employed to optimize synthesis conditions for the photocatalytic degradation of MB over the synthesized CeO 2/TiO2NTs, including four experimental parameters namely: hydrothermal temperature, hydrothermal time, CeO2/TiO2 molar ratio and calcination temperature Optimization results show that maximum removal yield (92.9 %) was obtained at the optimum synthesis conditions: hydrothermal temperature of 163 °C; calcination temperature of 557 °C; hydrothermal time of 20 hours and CeO2/TiO2 molar ratio of 0.1, The obtained results clearly demonstrated that response surface methodology (RSM) with a Box– Behnken design was one of the reliable methods for modeling and optimization of the synthesis variables 23 LIST OF ARTICLES RELATED TO DISSERTATION I Country Lê Thị Thanh Tuyền, Đào Anh Quang, Nguyễn Thị Quỳnh Trâm, Nguyễn Minh Quân, Trương Q Tùng, Trần Thái Hòa (2017), “Tổng hợp có kiểm sốt TiO2 cấu trúc ống nano quy trình thủy nhiệt”, Tạp chí Khoa học Cơng nghệ, Trường Đại học Khoa học – Đại học Huế, Tập 8, Số (2017), tr 109-121 Lê Thị Thanh Tuyền, Đào Anh Quang, Nguyễn Minh Quân, Nguyễn Thị Quỳnh Trâm, Trương Quý Tùng, Trần Thái Hòa (2017), “Tổng hợp đơn giản ống nano TiO2 pha tạp CeO2: ảnh hưởng tỉ lệ Ce:Ti nhiệt độ nung đến tính chất xúc tác quang”, Tạp chí Hóa học, Số 55 (4E23), tr 160-165 Lê Thị Thanh Tuyền, Đào Anh Quang, Nguyễn Minh Quân, Nguyễn Thị Vũ Tuyết, Trương Quý Tùng, Trần Thái Hòa (2017), “Tổng hợp ống nano TiO2 pha tạp CeO2 khảo sát hoạt tính xúc tác quang vùng ánh sáng khả kiến”, Tạp chí xúc tác hấp phụ, T6 (N0 1), tr 59-67 Lê Thị Thanh Tuyền, Đào Anh Quang, Trần Thanh Tâm Toàn, Trương Quý Tùng, Trần Thái Hòa (2018), “Các yếu tố ảnh hưởng đến phản ứng phân hủy quang hóa xanh methylene hệ xúc tác CeO2/TiO2 nanotubes”, Tạp chí Đại học Huế, Vol 127, No 1B (2018) II International (ISI) Le Thi Thanh Tuyen, Dao Anh Quang, Tran Thanh Tam Toan, Truong Quy Tung, Tran Thai Hoa, Tran Xuan Mau, and Dinh Quang Khieu (2018), “Synthesis of CeO2/TiO2 nanotubes and heterogeneous photocatalytic degradation of methylene blue”, Journal of Environmental Chemical Engineering , (2018), pp 5999-6011 24 ... Tổng hợp ống nano TiO2 pha tạp CeO2 khảo sát hoạt tính xúc tác quang vùng ánh sáng khả kiến , Tạp chí xúc tác hấp phụ, T6 (N0 1), tr 59-67 Lê Thị Thanh Tuyền, Đào Anh Quang, Trần Thanh Tâm Toàn,... phản ứng phân hủy quang hóa xanh methylene hệ xúc tác CeO2/ TiO2 nanotubes”, Tạp chí Đại học Huế, Vol 127, No 1B (2018) II International (ISI) Le Thi Thanh Tuyen, Dao Anh Quang, Tran Thanh Tam... XRD patterns of TiO2 synthesized at 160 oC for different hydrothermal time 3.1.2 Synthesis of CeO2/ TiO2- NTs Figure 3.3 XRD patterns of TiO2- NTs; TiO2- NTs 550; CeO2/ TiO2NTs@0.1 The XRD patterns