Oxidative removal of rhodamine B over Ti doped layered zinc hydroxide catalysts

Accepted Manuscript Title: Oxidative Removal of Rhodamine B over Ti-doped Layered Zinc Hydroxide Catalysts Author: Nguyen Tien Thao Doan Thi Huong Ly Han Thi Phuong Nga Dinh Minh Hoan PII: DOI: Reference: S2213-3437(16)30335-9 http://dx.doi.org/doi:10.1016/j.jece.2016.09.014 JECE 1256 To appear in: Received date: Revised date: Accepted date: 25-6-2016 18-8-2016 4-9-2016 Please cite this article as: Nguyen Tien Thao, Doan Thi Huong Ly, Han Thi Phuong Nga, Dinh Minh Hoan, Oxidative Removal of Rhodamine B over Ti-doped Layered Zinc Hydroxide Catalysts, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2016.09.014 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain Oxidative Removal of Rhodamine B over Ti-doped Layered Zinc Hydroxide Catalysts Nguyen Tien Thao*, Doan Thi Huong Ly, Han Thi Phuong Nga, Dinh Minh Hoan Faculty of Chemistry, Vietnam National University, Hanoi 19 Le Thanh Tong Street, Hanoi, Vietnam 100000 *Corresponding author: Tel (+84) – (093) 789 8917; Fax: (+84) – (04) 3824 1140 Email: ntthao@vnu.edu.vn (N Tien Thao) Graphical Abstract 100 1Ti-5Zn (Fresh) Degradation Efieciency (%) 90 80 70 3Ti-5Zn (Fresh) 60 50 40 30 1Ti-5Zn (Calcined) 20 3Ti-5Zn (Calcined) 10 0 10 Reaction time (h) Photodegradation efficiency of rhodamine B over Ti-Zn layered hydroxide catalyst uncalcined and calcined at 450 oC for hours under visible-light irradiation at room temperature Highlights The successful substitution of Zn2+ by Ti4+ in layered hydroxides/ Ti4+ in octahedral sites of hydroxide layers is prerequisite for the destruction of rhodamine B/ Effective degradation efficiency of rhodamine B at a near neutral pH conditions/ Abstract Ti-doped layered zinc hydroxide materials with different molar ratios of Ti/Zn have been synthesized through the coprecipitation method at pH of 9.0 - 9.5 The materials possess layered structure with carbonate anions in the interlayer regions The catalysts have uniform particle sizes and high surface area An isomorphous substitution of Zn2+ by Ti4+ in the brucite-like sheets makes a promotional effect on the photocatalytic activity in the degradation of rhodamine B aqueous solution The catalytic results indicated that the intra layer sheet Ti4+ ions are more active than the extra –TiO2 components in the complete removal of rhodamine B The degradation efficiency is dependant on the intra lattice Ti4+ contents and reaction variables Keywords: Ti-doping, degradation, rhodamine B, layered hydroxide, photocatalysis Introduction Photocatalysis has been well known as an advanced oxidation process used for the treatment of organic compound pollutants in water This is the most economic and promising industrial effluent treatment processes today [1-3] Among photocatalytic materials, semiconductor materials have been extensively studied for the effective removal of toxic organic dyes [4-8] And, TiO2-based material is one of the most popular semiconductors used as active heterogeneous catalysts for the wastewater treatment, air purification, water disinfection, hazardous waste remediation, and water purification [1,3,4] Titanium may be usually existed as TiO2 anatase dispersed on matrices, but it has a large band gap and thus its photocatalytic activity is usually limited to the UV region [2,3,6] Furthermore, the photocatalytic performance of TiO2 depends on its crystal structure, particle size as well as effective surface area [6-10] Therefore, a great effort of researchers has been made for the improvement of the TiO2 performances in the sunlight by loading on appropriate support, doping agents, and reducing TiO2 particle sizes [8,9,11,12] In practice, TiO2 nanoparticles have been recently shown excellent activity in the photocatalytic treatments of polluted water and the applications in nanotechnology [12-16] Nevertheless, the usage of nanosized TiO2 also faced numerous drawbacks such as hazardous human during the preparation and application [17] In other scenarios, TiO2 nanoparticles were easily aggregated, resulting in a rapid decrease of the overall activity after a short reaction period [16-19] Thus, researchers have been looking for other applicabilities of Ti-based heterogeneous catalysts Another promising way is to immobilize Ti4+ into the framework of structured materials such as TiO2 doped by noble metals [12,18], zeolite [20,21], and mesoporous materials [22-24] Indeed, titanium-substituted molecular sieves have shown good adsorption ability of dyes and catalytic activity in the decoloration of organic compounds in wastewater [20,22,24] For the purpose of the preparation of Ti-containing heterogeneous catalysts, we have incorporated some Ti4+ions into the layered zinc hydroxides It is well known that layered metal hydroxides contain a single type of metal cations with a positive charge usually accompanied by the appearance of hydroxyl vacancies [25-29] The representative hydroxide formulae are MII(OH)2n− y(X )x/n·mH2O (M = Zn2+, Cu2+, Ni2+) A typical example is a layered zinc hydroxide salt of Zn5(OH)8(Xn−)2/n·2H2O in which OH– deficiency on the host layers is compensated by coordinated guest anions (Xn-) [26,29,30] In the zinc hydroxide layers, one-quarter of octahedrally coordinated zinc ions is replaced by two tetrahedrally coordinated zinc ions located below and above the plane and water molecules are coordinated at the apexes of the tetrahedra Thus, they are widely used as inorganic adsorbents for the removal of anionic dyes or photocatalysts for the oxidation reaction [19,24,25] A substitution of Zn2+ by Ti4+ in the hydroxide layers leads to a variation in the solid composition formulated as [(Zn1−yTiy(OH)2)+y∙(X2y/nn−)2y−]∙mH2O and creates more OH- deficiencies in the hydroxide layers This modification allows to prepare several modified Zn-based catalysts with some desired photocatalytic properties due to the ability of hydroxide layers to accommodate some cations of various sizes and valences and the appearance of more foreign anions in the interlayer domains [14,25,26,32,33] In this work, the Zn–Ti LDHs have been synthesized by the co-precipitation method for the purpose of the preparation of effective catalysts used in the effective decomposition of rhodamine B in water under visible-light irradiation Experimental 2.1 Catalyst preparation Ti-doped layered zinc hydroxides were prepared by coprecipitation of the Zn(NO3)2·6H2O salt (Wako) with the desired amount of titanium isopropoxide (Merck) in a beaker containing 300 mL of solution of urea (1.45 M) heated at 90 oC (Table 1S in Supplementary Materials) The mixture was then stirred under reflux at 90 oC for 48 hours Under reported experimental conditions, the hydrolysis of urea results in the formation of ammonium cyanate and the resultant is further hydrolyzed into ammonium carbonate [14] As a consequence, the solution reaches a constant pH of - 9.5 due to the hydrolysis of ammonium and carbonate ions Then, the resultant slurry was then cooled to room temperature and separated by filtration, washed with hot distilled water several times until obtaining the neutral filtrate The filter-cake was then dried at 80 ◦C for 24 hours in oven For the sake of brevity, the prepared catalysts are denoted as xTi-yZn in which x/y is a molar ratio of Ti/Zn as reported in Table 2.2 Characterization Powder X-ray diffraction (XRD) patterns were recorded on a D8 Advance-Bruker instrument using CuKα radiation (λ = 0.1549 nm) Fourier transform infrared (FT-IR) spectra were obtained in 4000 – 400 cm-1 range on a FT/IR spectrometer (DX-Perkin Elmer, USA) UV–vis spectra were collected with UV-Visible spectrophotometer, JASCO V-670 BaSO4 was used as a reference material The spectra were recorded at room temperature in the wavelength range of 200-800 nm The scanning electron microscopy (SEM) images were obtained with a JEOS JSM5410 LV TEM images were collected on a Japan Jeol Jem.1010 The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method, and the pore size distribution and total pore volume were determined by the Brunauer–Joyner–Hallenda (BJH) method using a an Autochem II 2920 (USA) 2.3 Degradation of rhodamine B aqueous solution Rhodamine B (RhB) has been chosen as the degrading pollutions to test the photocatalytic activities of the as-prepared samples The catalyst of 0.3 g was dispersed in Rh B aqueous solution (100 mL, 20 ppm) and air was bubbled through the reaction mixture at the flowrate of about 5.0 mL/min A 40 Watt- visible- light -compact lamp was used as a light irradiation source and running water was circulated to ensure constant temperature of the reaction mixture, which was stirred magnetically The degradation efficiency of RhB was monitored using the UV-vis absorption spectra by measuring the peak value of a maximum absorption of rhodamine h B solution During the irradiation, about mL of suspension was continually taken from the reaction solution at given time intervals for the determination of rhodamine B concentration (C) The degradation efficiency (%) can be calculated as efficiency C0  Ct x100% where C0 is the initial concentration of dye C0 and Ct is the measured concentration of rhodamine B in aqueous solution at a given time (t) The concentration of the target dye is calculated by a calibration curve The maximum absorption of RhB was at a wavelength of 553 nm The UV-vis spectrophotometer (Shimazdu, Cary 100 UV-VIS spectrophotometer) with quartz cuvettes was used for the determination of color intensity in the range of 300-600 nm Calibration based on the Beer-Lambert law was used to quantify the dye concentration Results and Discussion 3.1 Characteristics of Catalysts XRD patterns for the synthesized solids with Ti4+/Zn2+ molar ratios of 0/5.0 – 3.0/5.0 and 1.0/6.0 in the preparation solutions were represented in Figure All samples show the observable reflection signals at 2-theta of 13.31, 28.37, and 38.89o corresponding to the basal planes (003), (006), and (009) respectively Furthermore, the additional weaker peaks at 2-theta of 31.01, 33.45, and 35.99o are respectively assigned to the reflections of some non-basal planes of (100), (101), and (012) All these diffraction peaks exhibit symmetric, strong peaks at low 2θ values and weaker, less symmetric peaks at high 2θ values which are typical characteristics for the layered-double hydroxide-type structure [14,30,31,33] Thus, XRD analysis indicates that the (Zn/Ti) layered double hydroxides have a lamellar structure in which Ti4+ ions were inserted into the brucite-like sheets of Zn-based layered materials [33,34] This is indeed confirmed by a small decrease of the thickness of the unit layer (d003) when the tetravalent metals (Ti4+) replace the divalent metals (Zn2+) in the framework of layered metal hydroxides (Table 1) As the layered hydroxide material is suggested to be a hexagonal close-packed structure, the cell parameter can be calculated using the equations c = 3d003 (three times the interlayer distance (0 3)) and a = 2d110 (average metal–metal distance in the interlayer structure (1 0)) The a cell parameters for a series of the synthesized samples increases from 3.106 to 3.182 Å as Ti/Zn molar ratio changes from 0/5 to 2/5 and the average cation-cation distance in the brucite-like sheets tends to decrease at a higher Ti/Zn molar ratio (Table 1) [34,35] In theory, a high amount of Ti4+ causes a higher density of tetravalent cations in the layers, resulting in the stronger electrostatic repulsion between positive charges Thus, there is only a certain amount of Ti4+ ions existed in the brucite-like sheets [35-37] Thus, a slight decrease of the parameter c value can be attributed to the stronger electrostatic interactions between Ti4+ cations and carbonate anions when Ti4+ ions are introduced in the lattice framework In principle, the charge density on the brucite-like sheets increases with the insertion of the Ti4+ ions, giving rise to a higher amount of carbonate anions required to maintain the electro-neutrality of the final material [17,36] However, the amount of carbonate anions in the Ti-richer samples (Table 1) is lower than the theoretical amount necessary for the charge neutralization because a fraction of Ti4+ species presents as the extra-framework hydroxide components in the Ti-richer samples (2Ti5Zn, 3Ti-5Zn) [14,33,38] Figure also exhibits a weaker intensity of the peak of 13.27o and the low signal-to-noise ratio for the Ti-rich samples, which is interpreted by the lower crystallinity degree of the corresponding solids Furthermore, the peak at 2-theta of 13.27o is vanished as calcination of Ti-doped samples at 450 ◦C and new peaks at higher 2θ values were observed (Figure 1S) A series of 2-theta peaks at 31.8, 34.5, 36.3, 47.5, 56.8, 62.8, 57.9o are solely attributed to the reflections of zinc oxide phase (JCPDS Card No 00-036-1451) and the observable 2-theta peak at 25.5o is likely attributed to the TiO2 anatase (JCPDS Card No 00-021-1272) on the calcined Ti-rich samples [29,32,33,39,40] (see Figure 1S in Supplementary Materials) Figure shows the UV-vis spectra of three Zn-Ti layered hydroxides in the region of 220– 800 nm The most intense band was observed at 305 nm, characterizing for Ti atoms in the hydroxide –like sheets [34-36] The broad adsorption band with maxima at 260 nm corresponds also to the Ti4+ species in an octahedral coordination The intensity absorption at higher wavelength (between 320–360 nm) can be due to the presence of the Zn2+ cations in the layered hydroxide solids as well as the presence of some Ti atoms in an octahedral environment participating in Ti–O– Ti bonds as part of small TiO2 particles [33,35] This absorption band slightly shifts to a longer wavelength as increasing amount of titanium A band at 216 nm is firmly assigned to the ligandmetal- charge transfer characterizing isolated Ti4+ ions in an octahedral environment FT-IR technique has been used to identify the nature and symmetry of interlayer anions and the metal-oxygen bonding Figure displays the FT-IR spectra for some Ti-doped layered zinc hydroxide catalysts before and after photocatalytic reactions It is noted all IR spectra are similar shapes, but the peak intensities of the spent catalysts are much lower than those of the fresh samples The broad intense bands between 3600 and 3200 cm-1 are ascribed to the OH stretching mode of layer hydroxyl groups and of interlayer water molecules [28,34,37] A weak shoulder peak recorded around 2920 cm-1 is ascribed to the OH stretching mode of interlayer water molecules [37,40] Simultaneously, the band at 1510 cm-1 is ascribed to the bending mode of water molecules In a lower wavenumber region, the band at 1365 cm-1 is assigned to the asymmetric stretching mode of the carbonate species [29,34,37,40] A weak band observed 831 cm-1 could be ascribed to out-ofplane bending vibration mode of carbonate anions It is observed some differences in signal intensities between the Ti-richer and -poorer samples probably due to different orientations of carbonate anions in the interlayer gallery as a result of different electrostatic forces in Ti-substituted samples The sharp band observed at 465 cm-1 for Ti–O–Zn in the framework of the solids [29,32,33] The catalyst morphology was investigated by SEM and TEM techniques Figure represents SEM micrographs of all Ti-Zn samples and a TEM image of a selected 1Ti-5Zn layered hydroxide material SEM micrograph of 1Ti-5Zn shows the presence of roughly hexagonal particles [20,27,36] These uniform primary particles aggregate into larger disk-like platelets with the mean crystal domain of 100-200 nm [17,28,36] In other context, TEM image of 1Ti-5Zn presents lamellar structure which is essentially characteristic for layered materials and the hexagonal packing structure (Fig 4B) [39,40] The primary particles with diameter ranging from 20 to 50 nm are clearly observed For the Ti-richer samples, the small particles agglomerate together to form larger plates (Fig 4) Thus, it is expected that there are many slit-shaped spaces between catalyst particles In practice, nitrogen adsorption/desorption measurement for some representative samples shows isothermal curves with a plateau from to 0.5 and a hysteresis loop in the range of 0.55 – 0.95 (Fig 2S) The patterns are likely classified to the IV type and the hysteresis loops are closely to the H3-classification, suggesting that these solids are either mesopores or nonporous materials [28,30,41] In the present work, the H3-like hysteresis 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Ping-Ping Qian, Ti-based layered double hydroxides: Efficient photocatalysts for Azo dyes degradation under visible light, Appl Catal B 144 (2014) 570-579 19 Table Caption Table Catalyst characteristics of all samples Figure Captions Figure Powder-XRD patterns of Ti-Zn Layered Double Hydrotalcite samples Figure UV-vis spectra of Ti-Zn Layered Hydrotalcite samples Figure FT-IR spectra of fresh and reacted Ti-Zn layered hydroxide catalysts Figure SEM (A-C) micrographs of Ti-Zn layered hydroxides and TEM (D) image of 1Ti-5Zn sample Figure EDS spectra of Ti-Zn layered hydroxide samples Figure Photodegradation efficiency of RhB during visible light irradiation or in dark over catalysts (20 ppm of rhodamine B, 0.30 grams of catalyst, room temperature, pH = 6) Figure 7A Photodegradation efficiency of RhB monitored as the normalized concentration changes with visible-light irradiation over Ti-Zn layered hydroxide catalysts (20 ppm of rhodamine B, 0.30 grams of catalyst, room temperature, pH = 6) Figure 7B UV-vis absorption spectra of rhodamine B aqueous solution during visible light irradiation over 1Ti-5Zn sample (20 ppm of rhodamine B, 0.30 grams of catalyst, room temperature, pH = 6) Figure Photodegradation efficiency (A) of RhB during visible light irradiation over 3Ti-5Zn layered hydroxide catalyst uncalcined and calcined at 450 oC for hours ([RhB] = 20 ppm, 0.30 grams of catalyst, visible-light irradiation, pH = room temperature) and UV-vis spectra (B) of the reaction solution Figure Effect of pH on the decoloration of rhodamine B, [RhB] = 20 ppm, 0.30 grams of 1Ti5Zn layered hydroxide catalyst, 28oC Figure 10 Effect of catalyst dosage on the decoloration of rhodamine B, (20 ppm of rhodamine B, 0.30 grams of catalyst, room temperature, pH of 6) Figure 11 Effect of reaction temperature on the decoloration of rhodamine B over 1Ti-5Zn sample (20 ppm of rhodamine B, 0.30 grams of catalyst, room temperature, pH of 6) Figure 12 Degradation of rhodamine B over reused Ti-Zn LDH catalyst at 28 oC, [RhB] = 20 ppm, 0.3 grams of catalyst, visible-light irradiation, pH of 20 Table Catalyst characteristics of all samples Batch # Theoretical molar ratio of Ti/Zn a (Å)* c (Å)* Ti Zn C SBET (At.%) (At.%) (At.%) (m2/g) 0Ti-5Zn 0/5 3.106 20.418 - - - 108.3 0.5Ti-5Zn 0.05/5 3.174 20.298 9.68 68.76 4.83 105.6 1Ti-5Zn 1/5 3.182 20.196 18.33 67.45 3.07 106.7 2Ti-5Zn 2/5 3.182 20.349 30.04 57.17 2.73 104.2 3Ti-5Zn 3/5 3.180 20.346 37.73 49.75 2.62 129.7 1Ti-6Zn 1/6 3.100 20.157 15.97 68.44 4.10 98.9 *a is an average metal–metal distance in the interlayer structure (110) and c corresponds to three times the interlayer distance (003) 21 (003) (100) (012 (101) (006) (113) (110) (009) 1Ti-6Zn 3Ti-5Zn 2Ti-5Zn 1Ti-5Zn 0.5Ti-5Zn 0Ti-5Zn 10 15 20 25 30 35 40 45 2-theta (o ) 50 55 60 65 70 Figure Powder-XRD patterns of Ti-Zn Layered Hydrotalcite Samples 22 3Ti-5Zn 2Ti-5Zn 1Ti-5Zn 0Ti-5Zn 200 250 300 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) Figure UV-vis spectra of Ti-Zn Layered Hydrotalcite samples 1Ti - 6Zn 0.5Ti - 5Zn 1Ti - 5Zn 2Ti - 5Zn 1Ti - 5Zn-Reacted 400 800 1200 1510 1365 831 465 2Ti - 5Zn-Reacted 1600 2000 2400 2800 3200 3600 4000 -1 Wavenumber (cm ) Figure FT-IR spectra of fresh and reacted Ti-Zn layered hydroxide catalysts 23 A: 1Ti-5Zn B: 2Ti-5Zn D: 1Ti-5Zn C: 3Ti-5Zn Figure SEM (A-C) micrographs of Ti-Zn layered hydroxides and TEM (D) image of 1Ti-5Zn sample 24 Figure EDS spectra of Ti-Zn layered hydroxide samples 1Ti-5Zn: White light 1Ti-5Zn: Dark Blank test (White light) 100 0Ti-5Zn: White light 0Ti-5Zn: Dark TiO2 (white light) Degradation Efficiency (%) 90 80 70 60 50 40 30 20 10 0 Time (h) Figure Photodegradation efficiency of RhB during visible light irradiation or in dark over catalysts (20 ppm of rhodamine B, 0.30 grams of catalyst, room temperature, pH = 6) 25 100 A Decoloration Efficiency (%) 90 80 70 60 1Ti-5Zn 50 40 0.5Ti-5Zn 30 3Ti-5Zn 20 2Ti-5Zn 10 1Ti-6Zn 0 Reaction time (h) 10 12 14 Figure 7A Photodegradation efficiency of RhB during visible-light irradiation (20 ppm of rhodamine B, 0.30 grams of catalyst, room temperature, pH = 6) 0h 2h 4h 6h 8h 10h 12h 14h 556 B 534 520 510 498 352 300 350 400 450 500 550 600 Wavelength (nm) Figure 7B UV-vis absorption spectra of rhodamine B aqueous solution during illumination over 1Ti-5Zn sample (20 ppm of rhodamine B, 0.30 grams of catalyst, room temperature, pH = 6) 26 100 A 1Ti-5Zn (Fresh) Degradation Efieciency (%) 90 80 70 3Ti-5Zn (Fresh) 60 50 40 30 1Ti-5Zn (Calcined) 20 3Ti-5Zn (Calcined) 10 0 10 11 Reaction time (h) 3.5 0h 2h 4h 6h 8h 10 h 2.5 B 1Ti-5Zn Calcined at 450oC 1.5 0.5 300 350 400 450 Wavelength (nm) 500 550 600 Figure Photodegradation efficiency (A) of RhB over Ti-Zn layered hydroxide catalyst uncalcined and calcined at 450 oC for hours ([RhB] = 20 ppm, 0.3 grams of catalyst, visible-light irradiation, pH = room temperature) and UV-vis spectra (B) of the reaction solution 27 100 A Decoloration Efficiency (%) 90 80 70 60 pH = 50 40 pH = 30 pH = 11 20 pH = 10 0 Reaction time (h) Figure Effect of pH on the decoloration of rhodamine B, [RhB] = 20 ppm, 0.30 grams of Degradation Eficiency (%) 1Ti-5Zn layered hydroxide catalyst, 28oC 100 90 80 70 60 50 40 30 20 10 0.5Ti-5Zn 0.4 g 0.3 g 0.2 g 0.1 g 10 Reaction time (h) Figure 10 Effect of catalyst dosage on the decoloration of rhodamine B, (20 ppm of rhodamine B, 0.30 grams of catalyst, room temperature, pH of 6) 28 A Degradation Eficiency (%) 100 80 60 40 318 K 20 301 K 278 K 0 350 400 450 10 B 1Ti-5Zn 0h 2h 4h 6h 8h 10 h 300 Reaction time (h) 500 550 600 Wavelength (nm) Figure 11 Effect of reaction temperature on the decoloration of rhodamine B (A) and UV-Vis absorption spectra of rhodamine B aqueous solution (B) during illumination over 1Ti-5Zn sample at 318K (20 ppm of rhodamine B, 0.30 grams of catalyst, pH of 6) 29 1Ti-5Zn 0.5Ti-5Zn 100 Degradation Efiency (%) 90 80 70 60 50 40 30 20 10 Number of cycles Figure 12 Degradation of rhodamine B over reused Ti-Zn LDH catalyst at 28 oC, [RhB] = 20 ppm, 0.30 grams of catalyst, visible-light irradiation, pH of 30 ... successful substitution of Zn2+ by Ti4 + in layered hydroxides/ Ti4 + in octahedral sites of hydroxide layers is prerequisite for the destruction of rhodamine B/ Effective degradation efficiency of rhodamine. .. Ti- ions and TiO2 particles in the decoloration of rhodamine B 3.2 Oxidative removal of rhodamine B aqueous solution The catalytic oxidation of rhodamine dye aqueous solution has been carried... Beer-Lambert’s law) was taken as the initial concentration of the dye for all the catalyzed reactions 3.2.1 Catalytic oxidative removal of rhodamine B over Ti- Zn layered hydroxide catalysts The rhodamine