Catalytic activity of TiO2/sepiolites in the degradation of rhodamine B aqueous solution - Nguyễn Tiến Thao

Vietnam Journal of Chemistry, International Edition, 55(2): 183-187, 2017 DOI: 10.15625/2525-2321.2017-00441 183 Catalytic activity of TiO2/sepiolites in the degradation of rhodamine B aqueous solution Nguyen Tien Thao 1* , Doan Thi Huong Ly 1 , Dinh Minh Hoan 1 , Han Thi Phuong Nga 1,2 1 Faculty of Chemistry, VNU University of Science – Vietnam National University, Hanoi 2 Faculty of Environment, Vietnam National University of Agriculture Received 7 July 2016; Accepted for publication 11 April 2017 Abstract TiO2/sepiolite catalysts were prepared by suspension of titanium dioxide and support in solvent accompanying by calcination. The characterization of the obtained powder has been examined by some physical means including XRD, SEM, FT-IR, and UV-vis. The sepiolite support possesses fibrous structure. X-ray diffraction analysis pointed out that the TiO2 particles are firmly distributed on fibrous sepiolite matrix. All TiO2/sepiolite samples were tested for the degradation of rhodamine B and showed a high catalytic activity. The experimental data showed that the degradation efficiency of rhodamine B is correlated with the amount of TiO2 loadings and oxidant behavior. At room temperature, the conversion of rhodamine B reaches to 99-100 % over 6.0 wt% TiO2/sepiolite catalyst. Keywords. TiO2, rhodamine B, sepiolite, degradation, photocatalysis. 1. ITRODUCTION The development of economy and industry in Vietnam also leads to some environmental issues during the last decades. A large quantity of organic contaminants in wastewater was exhausted into environments [1, 2]. Many of them are highly chemically stable, low biodegradable, and potentially harmful to the human society. As Law on Environmental Protection came ỉnto effect from January 01, 2015 in Vietnam, all toxic contaminants in exhausted wastewater must be treated before releasing water into rivers, fields, etc. Organic dyes and colored compounds are the source of considerable water consumption and contamination. Thus, the complete oxidation of these dyes in their aqueous solutions offers an opportunity of direct removal of these chemicals or their transformation into non-toxic products [2, 3]. However, efficiency of the classical oxidation processes for their removal from wastewater is still limited. For this reason, new advanced oxidation techniques are quite promising. They use active catalysts activated by sunlight irradiation for the dye degradation under ambient conditions [2-4]. Among heterogeneous photocatalysts used, TiO2 is reported as an effective semiconductor catalyst for removing stable organic compounds [3-5]. However, its catalytic activity sometimes varies with light frequency, phase, particle domain, dispersion... [4, 5]. Thus, distribution of TiO2 on matrix leads to an increased dispersion of active centers and improves catalytic activity. Among various inorganic materials reported, sepiolite a clay mineral having a unique structure related to its functional properties and adsorbability [6, 7]. Many works have reported the potential adsorption ability of dyes on this clay [8- 10]. This adsorptive property is an advantage to exploit its catalytic activity if this material is consisted of active components such as ZnO, FeOx, and TiO2. The purpose of the present study is to prepare TiO2 on fibrous sepiolite carrier as catalysts for the oxidation of rhodamine B. 2. EXPERIMENTAL 2.1. Catalyst preparation and characterization Sepiolite was purchased from Fluka Chemical Company and used without further purification. TiO2 was purchased from Wako Company. A certain amount of TiO2 was added into 25 mL of absolute ethanol under magnetic stirring at room temperature. The suspension was stirred for 10 minutes prior to adding a weighted quantity of dried sepiolite. The mixture was further stirred at room temperature for 3 hours and then evaporated at 70-75 o C for 15 hours VJC, 55(2), 2017 Nguyen Tien Thao et al. 184 to the yield white powder. The solid was then calcined at 400 o C for 2 hours to give TiO2/sepiolite samples. Powder X-ray diffraction (XRD) patterns were recorded on a D8 Avance-Bruker instrument using CuKα radiation (λ = 1.59 Å). Fourier transform infrared (FT-IR) spectra were obtained in 4000-400 cm -1 range on a FT/IR spectrometer (DX-Perkin Elmer, USA). The scanning electron microscopy (SEM) microphotographs were obtained with a JEOS JSM-5410 LV. UV–Vis spectra were collected with UV-Visible spectrophotometer. 2.2 Degradation of rhodamine B In photocatalytic experiments, 75 mL solution of 20 ppm of rhodamine B dye (RhB) and 0.45 grams of catalyst were added in to a beaker under magnetic stirring at room temperature. Then, either 75 mL solution of H2O2 (30%) was dropwised into the beaker or 5.0 mL/min flowrate of air was bubbled into the reaction mixture 2-5 mL of dye samples were taken out at a regular interval (20 min) from the solution test, filtered and their absorbance was recorded at 553 nm using a CARY 100 UV-vis spectrophotometer (Shimadzu). The degradation level is estimated by the following equation: 100 [RhB] RhB][[RhB] nDegradatio init ial finalinit ial 3. RESULTS AND DISCUSSION 3.1 Catalyst Characterization All TiO2/sepiolite samples with different loadings were prepared and their XRD patterns were represented in figure 1. As seen in figure 1, the reflection signals at 2-theta of 20.6, 23.8, 26.7, 28.0, 35.6, 37.9, 39.9, 43.8 o are indexed to the sepiolite phase (Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 00-013-0558) [6, 7, 9]. Some weaker signals at 2-theta of 25.5, 37.8, 55.1 o are essentially assigned to the TiO2 anatase (CPJS 00-021-1272). These peaks are rather broadening, implying the formation of nanocrystalline titanium dioxide loaded on the support [4, 5, 11]. 20 25 30 35 40 45 50 55 60 2-Theta ( o ) 2 wt.%TiO2/Sepiolte 6%TiO2/Sepiolte 15 wt.%TiO2/Sepiolte * * * Sepiolite Figure 1: XRD patterns for TiO2/sepliolite catalysts Morphology and microstructure of the raw sepiolite and TiO2/support are observed using scanning electron microscope and their micrographs are displayed in Fig. 2. The solid is consisted of a stick-like aggregation made up of lots of fibers and the length of sticks is approximately 1μm. The diameter of sticks is about 80 nm [12, 13]. No remarkable changes in the shape and size of Mg-O- Si sepiolite fibers were observed for the TiO2 loading samples (Fig. 2B). Figure 2: SEM images of sepiolite (A) and sample 15.0 wt% TiO2/sepiolite (B) A B VJC, 55(2), 2017 Catalytic activity of TiO2/sepiolites in 185 40080012001600200024002800320036004000 Wavenumber (cm -1 ) Sepiolite 2wt%TiO2/Sepiolite 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 200 300 400 500 600 700 800 Wavelength (nm) A b so rb a n c e 2 wt.%TiO2/Sepiolite TiO2 Figure 3: IR spectra (left) and UV-spectra (right) of TiO2 and TiO2/sepiolite samples FT-IR spectra of raw sepiolite and the TiO2/support are illustrated in Fig. 3A. The weak bands at 3610 and 3415 cm -1 for the three samples are assigned to the stretching vibrations of hydroxyl groups in the octahedral Mg sheet and external surface [8, 12, 13]. The band at 1650 cm −1 is due to the bending vibration of O-H bond of chemisorbed water on the surface of the solids. The bands around 1026 and 472 cm -1 which originate from stretching of Si-O in the Si-O-Si groups of the tetrahedral sheet still exist, indicating that the basic structure of sepiolite is well preserved [12, 13]. Fig. 3A also indicates no significant difference between the spectra of the TiO2/clay before and after suspension of TiO2. Figure 3B presents the UV-Vis diffuse reflectance spectra of TiO2/sepiolite. It is observed that two samples show a similar wavelength of the adsorption edge at 392 nm (Eg ≈ 3.20 eV), in line with the theoretical value of TiO2 photocatalyts [5, 14, 15]. Thus, no chemical interaction between titania and sepiolite was observed. The results suggest that the TiO2/sepiolites have a suitable band gap for photocatalytic reactions [16]. 3.2. Degradation of rhodamine B The degradation of rhodamine B was investigated in water at room temperature, laboratory lamp-light with air flow rate or 30% H2O2 solution as oxidant. For a comparison a blank test was carried out under the same conditions and a small amount of rhodamine B was converted, confirming the stability of organic dye [10]. Figure 4A shows that TiO2 pure oxide was also tested for the removal oxidation of rhodamine B with air. It is not supervising to see a gradually increased degradation degree of rhodamine B with reaction time since TiO2 is a typical photocatalyst. Figure 4B displayed the temporal changes in UV-vis spectra of the rhodamine B in the solution with reaction time. 0 20 40 60 80 100 0 1 2 3 4 5 6 7 8 9 10 Time (h) D e g r a d a ti o n P e r c e n t, % 6wt%TiO2/Sepiolite 15wt%TiO2/Sepiolite 8wt%TiO2/Sepiolite TiO2 (Pure) Bank test (No Catalyst) A TiO2 Catalyst 300 350 400 450 500 550 600 Wavelength (nm) 0h 2h 4h 6h 8h 10h B Figure 4: Catalytic activity of TiO2/sepiolite samples (A) and UV-vis absorption spectra of rhodamine B during visible light irradiation over TiO2 pure catalysts (20 ppm of rhodamine B, 0.30 grams of catalyst, room temperature) VJC, 55(2), 2017 Nguyen Tien Thao et al. 186 A gradual decrease in the intensity of the strong absorption band with the peak maximum at 553 nm is observed during the photocatalytic degradation of RhB white no wavelength shift of the band at 553 nm, implying the de-ethylation process of rhodamine B over the catalyst (Fig. 4B) [4, 5, 17, 18]. However, the degradation efficiency of rhodamine B sharply goes up as TiO2 particles were dispersed on sepiolite support. Indeed, the three TiO2/sepiolite catalysts exhibit rather high photocatalytic activity as compared with that of TiO2 pure experiment (Fig. 4). Figure 4A shows that the degradation level reaches nearby 100 % after 4-8 hours on time. In order to expedite degradation process, air flowrate was replaced by H2O2 oxidant. The oxidation of rhodamine B aqueous solutions with H2O2 was carried out over TiO2/sepiolite catalyst under ambient conditions. The catalytic activity of rhodamine B discoloration is represented in Figure 5. All catalyst samples show good activity in the oxidation of rhodamine B by H2O2. The discoloration reaction occurs more quickly and the degradation efficiency of rhodamine B increases after initiating reaction as seen in Fig. 5 [2, 18-20]. Evidently, the degradation efficiency of rhodamine B goes linearly up during 50 minute-reaction period and then gradually approaches about 100 %. 80 82 84 86 88 90 92 94 96 98 100 20 30 40 50 60 70 80 90 100 110 120 Reaction time (min) D e g r a d a ti o n P e r c e n t, % 6 wt%TiO2/Sepiolite 8 wt% TiO2/Sepiolite 4 wt%TiO2/Sepiolite 10 wt% TiO2/Sepiolite Figure 5: Catalytic activity of TiO2/sepiolite samples in the degradation of rhodamine B in the presence of H2O2 at room temperature, 0.3 grams of catalyst, 20 ppm RhB Figure 5 also reveals the comparative activity among catalyst samples. As seen in Fig. 5. The catalytic activity can be arranged in order of 6.0 wt% TiO2/sepiolite ≥ 8.0 wt% TiO2/sepiolite > 4.0 wt% TiO2/Sepiolite > TiO2. A higher photocatalytic activity for TiO2/sepiolite is explained by the high dispersion of TiO2 on the sepiolite surface, which provides more available active sites for the photocatalytic reaction. Furthermore, sepiolite was known as a good adsorbent and thus the catalyst surface may be the accumulation of rhodamine B molecules [7-10]. As a result, rhodamine B molecules have more chances to reach active sites and are therefore decolorized into intermediates [17- 19]. However, a higher TiO2 loading may lead to form large crystallite titania clusters which cover the sepiolite surface and finally decrease the photocatalytic activity. This explained a lower catalytic activity on 8.0 wt% TiO2/sepiolite [3, 15]. 4. CONCLUSION Sepiolite was used as support for TiO2 catalysts in the oxidative removals of rhodamine B. The support has layered structure with fibrous morphology. TiO2 was distributed on the sepiolite through the suspension and calculation route. TiO2/sepiolite was an excellent catalyst for the photodegradation of rhodamine B in the presence of H2O2 or air. Under the same experimental conditions, H2O2 was more oxidative than air in the discoloration of rhodamine B. 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