Journal of Science: Advanced Materials and Devices xxx (2017) 1e9 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Advanced oxidation of rhodamine B with hydrogen peroxide over ZneCr layered double hydroxide catalysts Nguyen Tien Thao a, *, Han Thi Phuong Nga a, Nguyen Que Vo a, Hong Dieu Khanh Nguyen b a b Faculty of Chemistry, Vietnam National University Hanoi, 19 Le Thanh Tong Street, Hanoi, 100000, Viet Nam School of Chemical Engineering, Hanoi University of Science and Technology, Dai Co Viet Street, Hanoi, Viet Nam a r t i c l e i n f o a b s t r a c t Article history: Received 10 July 2017 Received in revised form 11 July 2017 Accepted 14 July 2017 Available online xxx Zn/Cr layered zinc hydroxide materials with different molar ratios of Cr/Zn have been synthesized through the coprecipitation method at pH of 9.0e9.5 At high Cr/Zn molar ratios of 0.5/1e1/3, the materials possess some layered structure with carbonate anions between the interlayer galleries The catalysts present uniform particle sizes and quite high surface area An isomorphous substitution of Zn2ỵ by Cr3ỵ in the brucite-like sheets makes the layered Cr-doped zinc hydroxides acting as catalysts for efficient oxidation of rhodamine B with H2O2 solution The experimental results indicated that the intra-lattice Cr3ỵ ions are more active than Cr2O3 components in the oxidative removal of rhodamine B The degradation efciency is dependent on the intra lattice Cr3ỵ contents and reaction variables The Cr/Zn LDH gave a high decolorization (99%) of rhodamine B at near neutral pH and room temperature © 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Degradation Rhodamine B Layered hydroxides AOP Zn/Cr materials Cr2O3 Introduction The organic dyes have been widely used in the textile industry for last decades Nowadays, they have also known as toxic and polluted compounds because of their high persistence in soil sediments and water resources These chemicals may be degraded to the carcinogens and toxic products under sunlight irradiation, affecting aquatic life and ecosystem Therefore, in recent years, there has been an increasing demand for the destruction of organic dyes dissolved in water [1e3] Traditionally, biological, physical, and chemical methods have been applied for the removal of dyes [2e7] While the biological degradation of these dyes still faces challenges, chemical process and physical adsorption are more attractive in the treatment of wastewater [5,6] However, the chemical method in many cases generates a huge amount of sludge disposal that requires the posttreatment [3À5,7] In context, traditional physical methods such adsorption, filtration… are easy to operate, but they are nondestructive processes and certainly transfer the organic pollutants in water into the solid waste only [4e6] * Corresponding author Fax: ỵ84 04 3824 1140 E-mail address: ntthao@vnu.edu.vn (N.T Thao) Peer review under responsibility of Vietnam National University, Hanoi Recently, advanced oxidation processes have paid more attention as efficient routes approaching a complete destruction of organic dyes Particularly, semiconductor based photocatalysts are of great interest for hazardous wastewater removal Numerous materials such as iron oxide [7,8], Nb-based oxides [9,10], Wcontaining materials [11,12], titanium dioxides and Ti-related catalysts [13e20] are known as good photocatalysts for the oxidation of organic pollutants, however, these solids can only be activated under UV-light irradiation because of their large band gap Therefore, these catalysts need to either modify for the utilization under visible light or undergo the advanced oxidation reactions in the degradation of organic pollutants [11,16,18,20] Layered double hydroxides (LDHs) are classified into a class of anionic mineral clays with the general formula expressed as 2ỵ xỵ n 2ỵ [M1x M3ỵ (Mg2ỵ, Ni2ỵ, x (OH)2] (A )x/n$mH2O, where the M 2ỵ 2ỵ 2ỵ 3ỵ 3ỵ 3ỵ 3ỵ Co , Cu or Zn ) and M (Al , Cr , Fe ) cations are divalent and trivalent metal ions positioned in the center of an octahedron and the AnÀ anions are occupied in the spaces between two octahedral layers [5,21e24] LDHs have been widely synthesized for the applications in basic catalysts [21,22], oxidation-reduction catalysts [24e27], adsorbents [28], anion exchangers [21,29], and so on Recently, LDHs have been shown a good photocatalytic activity for the degradation of organic compounds [23,25,27,30e32] For http://dx.doi.org/10.1016/j.jsamd.2017.07.005 2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Please cite this article in press as: N.T Thao, et al., Advanced oxidation of rhodamine B with hydrogen peroxide over ZneCr layered double hydroxide catalysts, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.005 N.T Thao et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e9 USA) Energy-dispersive spectroscopy (EDS) data were obtained from Varian Vista Ax X-ray energy-dispersive spectroscope example, Zn/Cr LDHs was reported to be good reactivity for the degradation of methylene blue under visible-light irradiation [32] Zn/Ti LDH catalysts showed a good activity of in the degradation of rhodamine B in the presence of air [31] Thus, layered hydroxide materials are expected to be potential catalysts for the applicability to oxidation of dyes in water In the present work, we report on the synthesis of layered double hydroxide containing both Cr and Zn in the brucite-like layers Indeed, a substitution of Zn2ỵ by Cr3ỵ in the zinc hydroxide layers leads to a variation in the solid composition formulated 2y as [(Zn1yCry(OH)2)ỵy$(Xn ]$mH2O and creates more OHÀ 2y/n) deficiencies in the hydroxide layers This modification allows preparing some modified Zn-based catalysts with some desired catalytic properties due to the ability of hydroxide layers to accommodate some cations of various sizes and valences The synthesized ZneCr LDHs are expected to be effective catalysts used in the oxidation of rhodamine B in water 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 0.30 g dispersed in Rh B aqueous solution (100 mL, 20 ppm) was dropped by H2O2 solution (30%) at the flow rate of about 1.0 mL/min The reaction was magnetically stirred under ambient condition (under 40 W-fluorescent-lamp in the laboratory, room temperature of 28 C) The degradation efficiency of RhB was monitored using the UVevis absorption spectra by measuring the peak value of a maximum absorption of rhodamine RhB 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 (Ct) The degradation efficiency (%) can be calculated as decoloration ¼ C0 À Ct/C0 Â 100% where C0 is the initial concentration of dye 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 UVevis spectrophotometer (Shimazdu, Cary 100 UVVIS spectrophotometer) with quartz cuvettes was used for the determination of color intensity in the range of 300e600 nm Calibration based on the BeereLambert law was used to quantify the dye concentration Experimental 2.1 Catalyst preparation ZneCr layered double hydroxides were prepared by coprecipitation of the Zn(NO3)2$6H2O salt (Wako) with the desired amount of Cr(NO3)3$xH2O (Wako) in a beaker containing 300 mL of solution of urea (1.45 M) heated at 90 C The mixture was then stirred under reflux at the same temperature for 48 h Under reported experimental conditions, the hydrolysis of urea results in the formation of ammonium cyanate and the resultant is further hydrolyzed into ammonium carbonate [31] As a consequence, the solution reaches a constant pH of 9e9.5 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 filtercake was then dried at 80 C for 24 h in oven For the sake of brevity, the prepared catalysts are denoted as CrZnx in which x is a molar ratio of Cr/Zn as reported in Table For a mixed oxide (reference) sample, two salts, Zn(NO3)2$6H2O, Cr(NO3)3$9H2O were separately calcined at 450 C in air for h, then two powder oxides were blended with Cr/Zn molar ratio of 1/ The reference sample is designated as MiOx Results and discussion 3.1 Characteristics of catalysts Some typical characteristics of the catalysts are summarized in Table X-ray diffraction patterns of all samples are reported in Fig The diffraction peaks of starting Zn(OH)2 matched with a standard pattern (JPCDS 00-048-1066) [33] After adding chromium(III) ions to the zinc hydroxide lattice, some new diffraction peaks indexed into the (003), (006), (012), and (110) planes typically characterize for the layered structure of the Cr-rich materials (x > 1/3) This result clearly demonstrated that the Zn(OH)2 lattices are rmly adopted a certain amount of Cr3ỵ to form a zinc chromium carbonate layered hydroxide (JPCDS Card No 00-052-0010) The CreZn layered hydroxide phase was detected as a dominant component at high concentration of Cr ions in the solids (sample CrZn1eCrZn2) while a mixture of zinc hydroxides and Cr-doped LDHs are present in the Cr-poor samples (x 1/3) [27,34] Furthermore, the diffraction peaks of layered hydroxides gradually become broader when the Cr content increases from sample CrZn5 to CrZn1, reflecting the formation of nanometric CreZn layered hydroxide crystals Such morphology is confirmed by microscopy techniques As seen in Fig 2, the fresh Cr-doped samples are composed of irregular-shaped nanoparticles with the mean size of 50e70 nm Meanwhile, the two Cr-poorer samples (CrZn3 and CrZn5) possess 2.2 Characterization Powder X-ray diffraction (XRD) patterns were recorded on a D8 Advance-Bruker instrument using CuKa radiation (l ¼ 0.1549 nm) The scanning electron microscopy (SEM) images were obtained with a JEOS JSM-5410 LV TEM images were collected on a Japan Jeol Jem.1010 The specific surface area was calculated by the BrunauereEmmetteTeller (BET) method, and the pore size distribution and total pore volume were determined by the BrunauereJoynereHallenda (BJH) method using an Autochem II 2920 (USA) Fourier transform infrared (FT-IR) spectra were obtained in 4000e400 cmÀ1 range on a FT/IR spectrometer (DX-Perkin Elmer, Table Some typical characteristics of all catalyst samples Batch # Theoretical formula of CreZn LDH samples Cr:Zn ratio Cr (wt.%) Zn (wt.%) SBET (m2/g) CrZn1 CrZn2 CrZn3 CrZn5 Zn(OH)2 MixO Zn1/2Cr1/2 (OH)2(CO3)1/4,xH2O Zn2/3Cr1/3 (OH)2(CO3)1/6,xH2O Zn3/4Cr1/4(OH)2(CO3)1/8,xH2O Zn5/6Cr1/6(OH)2(CO3)1/12,xH2O Zn(OH)2 Cr2O3 þ ZnO 1:1 1:2 1:3 1:5 e 1:2 20.78 13.17 10.17 6.48 e e 29.45 38.34 42.34 54.21 e e 148 154 139 134 108 87 Please cite this article in press as: N.T Thao, et al., Advanced oxidation of rhodamine B with hydrogen peroxide over ZneCr layered double hydroxide catalysts, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.005 N.T Thao et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e9 Fig Powder-XRD patterns of CreZn layered hydroxide samples some large nanoplates, in addition to numerous grains of layered hydroxide crystals Each individual plate has an average diameter of 300e400 nm and a length of microns, orients outward from center More plates are observed on the Zn-richer samples (Fig 2), which are described as the Zn(OH)2 component This observation is in good harmony with the TEM observation As seen in Fig 3, sample CrZn2 presents mainly thin sheet-shaped morphologies and the border lengths of these sheets are a few dozens of nanometers Fig SEM micrographs of CreZn layered hydroxides Please cite this article in press as: N.T Thao, et al., Advanced oxidation of rhodamine B with hydrogen peroxide over ZneCr layered double hydroxide catalysts, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.005 N.T Thao et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e9 Moreover, the TEM image also reveals planar structure which is typically characteristic for the layered hydroxide materials [21,27,31] As the Cr content decreases, the inhomogeneous shapedmorphologies are observed: a mixture of sheet-shaped and plateshaped crystals are clearly present in the samples (CrZn5) The mean crystal domain of the nano-sized palettes is around 80 nm while that of the large plates is in the scope of several microns [35,36] In addition, the aggregation of uniform nanoparticles leads to the formation of voids between grains, resulting in a large external surface area while the close-folded agglomeration of the zinc hydroxide plates leading to decrease of specific surface area of the solids (Table 1) The surface chemical composition of the synthesized CreZn LDH materials was ascertained using EDS technique Fig indicates that Zn, Cr, Cr, O, C all can be detected The molar ratio of Zn and Cr is close to the theoretic stoichiometric atomic ratio A small deviation in molar composition is attributed to the coprecipitation at pH constant and the inhomogeneous composition of the catalyst surface (Figs and 3) [30,36,37] Indeed, as seen in Fig 4, analysis of nitrogen physic sorption indicates the BrunauereEmmetteTeller (BET) surface area of all samples in the range of 100e154 m2/g The BET surface area of layered hydroxide samples is likely related to the composition of the solids Furthermore, Fig represents nitrogen adsorption/desorption isothermal curves of the catalysts belonging to the type IV according to the IUPAC classification The hysteresis loops with the sloping adsorption branch and nearly vertical desorption branch correspond to type H2, which is essentially characteristic for pore-like-shapes with empty spaces [35,38] The chemical bonding behavior of the Cr-doped layered zinc hydroxides is investigated by FT-IR spectroscopy Fig displays the FT-IR spectra for the fresh and some spent CreZn LDH samples No major difference in spectra between the fresh and spent samples is observed In both cases, it is observed that a very intense and broad absorption band around 3560 cmÀ1 is assigned the stretching mode of layer hydroxyl groups and the stretching mode of interlayer water molecules This broadening band in the lower wavenumbers is due to the hydrogen bonding between the water molecules and the interlayer anions in the CreZn LDH [21,35] Furthermore, a weak band around 1650 cmÀ1 is attributed to the bending mode of water molecules [26,35,39] A sharp band at 1380 cmÀ1 is solely assigned to the antisymmetrical stretching mode of the carbonate anions in interlamellar spaces [20,24,26,35,39] The absorption bands below 1000 cmÀ1 can be attributed to the MÀO vibration modes of the ZneCr patterns Thus, the presence of Cr3ỵ ions in the layered zinc hydroxides is expected to be active sites for the destructive oxidation of rhodamine B in water Fig TEM images (left) and EDS spectra (right) of CreZn layered hydroxide samples Please cite this article in press as: N.T Thao, et al., Advanced oxidation of rhodamine B with hydrogen peroxide over ZneCr layered double hydroxide catalysts, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.005 N.T Thao et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e9 Fig Nitrogen adsorption/desorption isothermal curves of CreZn layered hydrotalcite samples Fig FT-IR spectra of fresh and reacted ZneCr layered hydroxide catalysts: (a): CrZn2-Fresh; (b): CrZn2-Spent; (c): CrZn3-Fresh; (d): CrZn3-Spent; (e): CrZn5-Fresh (spent samples: catalysts were recorded FT-IR spectra after reaction) 3.2 Oxidative removal of rhodamine B aqueous solution The catalytic oxidation of rhodamine dye aqueous solution has been carried out using H2O2 aqueous solution at room temperature and atmospheric pressure All the experiments were performed at the pH around 6.5 with a catalyst amount of 0.30 g After a given time interval, the corresponding concentration of the dye (as measured by UV spectrophotometer and the concentration evaluated using BeereLambert's law) was taken as the measured concentration of the dye for all the catalyzed reactions 3.2.1 Catalytic degradation of rhodamine B with H2O2 The rhodamine B aqueous solution was stirred under white light over the CreZn LDH catalysts in the presence of H2O2 for catalytic reaction at room temperature Firstly, the control experiments were performed in the dark without H2O2 oxidant to determine the adsorption ability of RhB over the catalysts In the UVevis spectra of RhB solution, the absorbance of dye solution at 553 nm (n / p* transition of C]N, C]O groups) is used to monitor the decolonization level of the dye between time intervals [6,8,10,31,39] It is noted that when reaching equilibrium adsorption state under the dark conditions, the samples CrZn1eCrZn5 shows about 3.8e5.6% adsorption of RhB molecules Another experiment (blank test) was carried out under (laboratory) visible light and H2O2 in the absence of the catalysts for comparison Fig demonstrates that the blank test has no conversion of rhodamine B (see Fig 1S in Supplementary Materials) although a remarkable amount of H2O2 was added to the reaction mixture In other context, Zn(OH)2 and MiOx samples both exhibit a negligible amount of RhB removed from the solution (Figs 6b and 2S) Meanwhile, all ZneCr LDH catalysts show very high activity in the degradation of rhodamine B (Fig 6a), substantiating a crucial role of Cr-doped layered zinc hydroxides in the oxidation of the organic dye with H2O2 In a series of Crcontaining hydroxide catalysts, the catalytic activity decreases in the order of CrZn2 > CrZn5 > CrZn1 ~ CrZn3, which are believed to be correlated with the intra-lattice Cr3ỵ content and the catalyst texture As indicated in Fig 1, the sample CrZn2 exhibits mainly CreZn LDH phase while the others possess both CreZn LDHs and Zn(OH)2 constituents The catalytic activity of Cr3ỵ in the oxidation of rhodamine was corroborated by the observable changes in UVevis spectra of RhB in different reaction periods (Fig 2S-c, Supplementary Materials) It is clear to observe a rapidly decreased Please cite this article in press as: N.T Thao, et al., Advanced oxidation of rhodamine B with hydrogen peroxide over ZneCr layered double hydroxide catalysts, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.005 N.T Thao et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e9 Fig Degradation efficiency of RhB (a) during visible light irradiation (20 mg/L of rhodamine B, 0.30 grams of catalyst, room temperature, pH ¼ 6.5, H2O2 solution, laboratory light) after 210 and (b) UVevis absorption spectra of rhodamine B aqueous solution over three samples absorption intensity of RhB at 553 nm after 30 and then absorbance of RhB solution likely approaches the baseline after 210 of reaction These results re-affirm the promotional effects of Cr3ỵ in the zinc hydroxide lattice on the advanced oxidation of rhodamine B in the presence of H2O2 (Fig 2S) [31,40] Furthermore, no shift of adsorption wavelength in UVevis spectra (Fig 2S) suggests the simultaneous degradation of both the dye chromophores and the aromatic rings under these reaction conditions [12,20,31,40] A lower activity of the Zn-richer catalysts (CrZn3 and CrZn5) attributed to the existence of a remarkable amount of the zinc hydroxides in the catalysts (Fig 2) [25,30,31,33] The activity of CrZn1 is on a par with that of CrZn3, interpreting that the zinc hydroxides would only accommodate a certain amount of Cr3ỵ ions in the layered double hydroxide framework [30,35,37] All experiments have been carried out under the laboratory light exposure (40 W-fluorescent lamps); the photocatalytic reaction may be presumably activated In order to shred in light of the nature of catalytic reaction, an additional set of reactions was performed in dark under controlled conditions Fig depicts the catalytic activity versus reaction time and the temporal UVevis spectral changes of RhB aqueous solution during the catalytic degradation reaction (Fig 7b and c) It could be noted that no significant difference in catalytic activity tested in dark or in visible lighteexposure for 120 min-reaction-course, and all experiments show a high decoloration percent of RhB after 210 Furthermore, the UVevis spectra of these RhB solutions are almost similar (Fig 7b and c), referring that catalytic reaction was insignificantly affected by the fluorescent light source in the present work [2] Moreover, the temporal UVevis spectra of RhB aqueous solution (Fig 7) shows a gradual reduction on the absorbance at 553 nm only, confirming that the degradation of RhB proceeds via chromophore cleave, followed by some other reactions including hydroxylation, aromatic ring opening and mineralization [39e42] Therefore, it is suggested that the organic dye was eliminated by the advanced chemical oxidation process in the presence of CreZn LDH catalysts [30,40] Please cite this article in press as: N.T Thao, et al., Advanced oxidation of rhodamine B with hydrogen peroxide over ZneCr layered double hydroxide catalysts, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.005 N.T Thao et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e9 3.2.3 Effect of oxidant amounts Because H2O2 oxidant exhibits an excellent ability to decolorate rhodamine B aqueous solution over CreZn LDH catalysts, different amounts of H2O2 are added into the reaction mixture in order to optimize the reaction conditions Fig illustrates the degradation level of rhodamine B varying with the concentration of H2O2 in the range of 3e12 g/L The degradation level of rhodamine B changes from 10.2 to 99.1% after 120 min, depending on the amount of H2O2 present in the reaction mixture (Fig 9) At a low initial concentration of H2O2, the catalytic activity trend likely approaches a plateau at the decoloration of RhB around 18e22% because of limiting reagent of H2O2 in these experiments No doubt, the decoloration of RhB is proportional to the contribution of additional HO radicals produced from H2O2 dissociation as the H2O2 concentration increases [37,42] An increased H2O2 amount in the reaction solution, the decoloration sharply goes up and reaches approximately to 100% after 90 However, no observable differentiation of removal efficiency of RhB in the H2O2 initial concentration between and 12 g/L This phenomenon is explained by the fact possibility that the formation of HO2 radicals is resulted from an interaction between H2O2 and HO at high initial amount of hydrogen peroxide solution [12,37] Another possibility is that the reaction between HO2 and HO radicals to form H2O and O2 also leads to a major decreased concentration of HO in the reaction mixture on the CreZn based catalysts [10,12,31] As a consequence, a higher initial concentration of H2O2 would not facilitate the removal oxidative reactions of RhB on ZneCr LDH catalysts Fig Degradation efficiency (a) of RhB over ZneCr layered hydroxide catalysts ([RhB] ¼ 20 mg/L, 0.3 g of catalyst, pH ¼ 6.5, room temperature) and UVevis spectra (b) of the reaction solution in dark and (c) in visible light irradiation 3.2.2 Effect of the oxidant nature Since the CreZn LDHs exhibit a high activity in the oxidation of rhodamine B with H2O2 aqueous solution, the catalysts have been performed in the presence of oxygen/air under similar reaction conditions for comparison Fig displays the variation of the decoloration percent versus reaction times under two different oxidant agents Under similar reaction conditions, H2O2 can decolorize about 99% of rhodamine B after 1.5 h while air can only give 10% of rhodamine B after 14 h-on-time The experimental results also indicate that H2O2 is much reactive oxidant for the oxidation of RhB over ZneCr LDH catalysts, which may be related to the great ability of H2O2 to form HO radicals in the Cr-doped LDH catalyst bulk [8,20,31] Meanwhile, oxygen molecule has likely no impact on the oxidative removal of rhodamine B [7,31,41,42] The RhB decoloration level in the latter case is close to that of the blank testing reaction (Figs 6a and 7a), reflecting the no reactivity of oxygen oxidant in the degradation process of RhB on the catalysts 3.2.4 Effect of the catalyst dosages Since the amount of H2O2 in the reaction solution has some remarkable effects on the removal efficiency of rhodamine B, the catalyst dose may be an important factor influencing on the degradation of organic dyes [31,37] In the present study, the quantity of catalyst in the range of 0e3 g/L is examined at reaction conditions reported in Fig 10 Obviously, the decoloration trend of RhB varies proportionally with the catalyst doses Indeed, the catalytic activity is related to the availability of more active sites on catalyst surface and the amount of Cr-doped layered zinc hydroxide phase present in the catalyst sample As other transition metal ions located in the material framework [26,31,35,43], these results mean that Cr3ỵ ions adopted in the layered hydroxide sheets could be stabilized and efficiently proceed the decoloration of rhodamine B The highest decolorization was observed with 3.0 g/L and thereafter increase in material dosage had significant effect on degradation of RhB in the examined catalyst dosages [10,12,30,37] Conclusion The structure and morphology of the synthesized ZneCr layered hydroxides depend on the preparation conditions and Cr/Zn molar ratios At a Cr/Zn molar ratio of ½, the CreZn LHD gave layered structure with carbonate anions in the interlayer regions and uniform particle sizes and high external surface areas while other Cr/ Zn ratios result in a mixture of oxides and layered hydroxides The CreZn layered hydroxide catalyst showed a good activity in the advanced oxidation of rhodamine B with H2O2 at a near neutral pH The intra-layer Cr3ỵ ions are active sites for the complete destruction of rhodamine B while the extra-hydroxide framework chromium oxide exhibited a negligible decoloration level of RhB The degradation efficiency depends on the intra-layered hydroxide Cr3ỵ contents and reaction variables The highest degradation efficiency of rhodamine of 99.8% was observed on sample ZnCr2 after 90 Our findings indicate that CreZn layered hydroxide Please cite this article in press as: N.T Thao, et al., Advanced oxidation of rhodamine B with hydrogen peroxide over ZneCr layered double hydroxide catalysts, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.005 N.T Thao et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e9 Fig Effect of oxidants on the decoloration of rhodamine B, [RhB] ¼ 20 mg/L, 0.30 g of CrZn2 layered hydroxide catalyst, room temperature, air flowrate of mL/min for 14 h Fig Effect of H2O2 amounts on the decoloration of rhodamine B, [RhB] ¼ 20 mg/L, 0.30 g of CrZn2 layered hydroxide catalyst, pH ¼ 6.5, room temperature Fig 10 Effect of catalyst dosage on the decoloration of rhodamine B, (sample CrZn3, 20 mg/L of rhodamine B, 0.30 g of catalyst, room temperature, pH of 6.5, room temperature) Please cite this article in press as: N.T Thao, et al., Advanced oxidation of rhodamine B with hydrogen peroxide over ZneCr layered double hydroxide catalysts, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.005 N.T Thao et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e9 catalysts are promising catalysts for degradation of organic dyes in water under an ambient condition Acknowledgment This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.05e2017.04 Appendix A Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jsamd.2017.07.005 References [1] Olivier Lefebvre, Rene Moletta, 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Table Some typical characteristics of all catalyst samples Batch # Theoretical formula of CreZn LDH samples Cr :Zn ratio Cr (wt.%) Zn (wt.%) SBET (m2/g) CrZn1 CrZn2 CrZn3 CrZn5 Zn( OH)2 MixO Zn1 / 2Cr1 /2... presence of CreZn LDH catalysts [30,40] Please cite this article in press as: N.T Thao, et al., Advanced oxidation of rhodamine B with hydrogen peroxide over ZneCr layered double hydroxide catalysts, ... article in press as: N.T Thao, et al., Advanced oxidation of rhodamine B with hydrogen peroxide over ZneCr layered double hydroxide catalysts, Journal of Science: Advanced Materials and Devices (2017),