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ZrO2 photocatalyst derived from zirconium metal organic framework for degradation of organic pollutants under visible light irradiation
Journal of Environmental Chemical Engineering 7 (2019) 103096
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Synthesis of porous TiO2/ZrO2 photocatalyst derived from zirconium metal organic framework for degradation of organic pollutants under visible light irradiation ⁎
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Jafar Abdi , Maede Yahyanezhad, Sahar Sakhaie, Manouchehr Vossoughi , Iran Alemzadeh Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Metal-organic frameworks Photocatalyst Porous TiO2/ZrO2 Synthesis Wastewater treatment
Zirconium based metal-organic frameworks (Zr-MOFs) are promising candidates for photocatalytic wastewater treatment due to their excellent properties such as high chemical and thermal stability and high photodegradation ability. Herein, we report a novel porous TiO2/ZrO2 photocatalyst derived from UiO-66 and Titania hybrids. UiO-66 nanoparticles was synthesized through solvothermal method and utilized as catalyst support to grow TiO2 particles on its surface. The prepared Titania/MOF nanocomposite was calcined to obtain porous TiO2/ZrO2 photocatalyst for degradation of organic pollutants from colored wastewater under LED visible light. The prepared materials were fully characterized with FTIR, XRD, SEM/EDS, TEM, BET, UV-DRS and ICP analysis. The results showed that the developed TiO2/ZrO2 enhanced photodegradation ability of Rhodamin B (RhB) in comparison with the mixture of prior UiO-66 and TiO2 and was found to affect the photocatalytic activity by increasing the adsorption of photons in visible region and enhanced the transfer and separation of produced charge. The decolorization kinetics followed first-order kinetic model. In addition, after four times recycling, the regenerated nanocomposite still showed high stability and photodegradation ability (90%).
1. Introduction In recent years, water pollution and the crisis of water shortage, are the most important problems that humans have been faced. A great amount of water pollution comes from chemical industries such as textile or paper industry [1]. Wastewater treatment of textile industry is the most difficult one among others since textile industry utilizes more than 7 × 105 tones dyes annually and 2% of the amount of dyes, which has been used, is discharged into the effluent and 10% is lost during the dyeing processes [2,3]. Removal of dyes form colored wastewaters is more essential because a small amount of dyes in wastewater is harmful for humans’ environment and ecosystem [4]. Furthermore, in comparison to other contaminants, dyes are more resistant to conventional methods of wastewater treatment. Several methods such as physical, biological and chemical processes are employed for water purification and removal of different contaminants from wastewaters [5,6]. Among different treatment methods, chemical treatments especially advanced oxidation processes (AOPs) counts as one of the most effective methods in dye removal from wastewater [7,8]. Some advantages of AOPs are that these processes
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operate at or near ambient temperature and pressure and convert approximately all the organic compounds contaminating effluents into less hazardous products [9]. Processes using H2O2 or UV irradiation [10], Fenton and photo-Fenton catalytic reactions as some examples of AOPs [11], are efficacious methods based on the generation of hydroxyl radicals which oxidizes a wide range of organic pollutants in wastewater non-selectively. Using photocatalytic degradation processes under UV/visible irradiation is a newborn and green method in wastewater treatment especially in the field of dye degradation. Several photocatalysts such as TiO2, ZrO2, ZnO, etc. have been used for destroying pollutants [12–15]. Among different photocatalysts, TiO2 has attracted a great attention due to the fact that it is not toxic and harmful for the environment and has a low-cost synthesis. Besides, there are many studies on TiO2 among other photocatalyst; Although, the band gap of TiO2 is relatively wide (3.2 eV) which makes it not effective enough under visible light [15]. However, whether semiconductors combined with some other porous materials may enhance the photocatalytic performance and make the photocatalytic processes more sufficient. One of the new emerged compounds named metal organic frameworks (MOFs) have attracted
Corresponding author. Corresponding author. E-mail addresses: [email protected], [email protected] (J. Abdi), [email protected] (M. Vossoughi).
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https://doi.org/10.1016/j.jece.2019.103096 Received 19 March 2019; Received in revised form 11 April 2019; Accepted 13 April 2019 Available online 16 April 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103096
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Nomenclature UV LED FTIR SEM TEM XRD
BET EDS DRS DMF BDC HCl NaOH
Ultra violet Light-emitting diode Fourier transform infrared Scanning electron microscopy Transmission electron microscopy X-ray diffraction
significant attention as a photocatalyst [16,17]. MOFs as organic-inorganic hybrid materials are a new kind of porous compound which have been surveyed in several fields such as catalysis [18–20], gas storage [21], sensors [22], chemical separation [23], adsorption [16], etc. These materials have attracted great attention because of their high surface area, high porosity, and low density [24]. Several studies have been conducted on MOFs, results of which have shown that these structures can act as a photocatalyst under UV or visible irradiation in Rhodamin B (RhB) degradation process [25–29]. As one of Zr-based MOFs, UiO-66, possesses high thermal and mechanical stability [30]. Nevertheless, the band gap of UiO-66 is about 3.6 eV which limit its optical adsorption in the visible light region. To achieve this, a narrow band gap semiconductors coupling formed a heterojunction method is an ideal choice. The heterojunction can form an inner electric field to facilitate the transfer/separation of photogenerated electron/hole and inhibit the recombination of electrons and hole so as to enhance the photocatalytic activity [31]. So, it is rational to expect that by hybridizing UiO-66 with another excellent semiconductors, a more stable and efficacious MOF composite with photocatalytic activity would be attained. In this study, we are reporting porous TiO2/ZrO2 derived Titania/ MOF nanocomposite as a photocatalyst for degradation of RhB under LED visible irradiations for the first time. The functional group, crystalline structure, morphology and photocatalytic activity of prepared material was investigated by FTIR, XRD, SEM/EDS, TEM, BET, UV-DRS, and ICP analysis. The results showed that incorporation of Zr-MOF and TiO2 increased photodegradation ability of RhB. Effects of different factors such as initial solution pH, amounts of photocatalyst and dye concentration were investigated in dye degradation process.
Brunauer-Emmett-Teller Energy-dispersive X-ray spectrometry Diffuse reflectance spectroscopy Dimethylformamide Benzene-dicarboxylic acid Hydrochloric acid Sodium hydroxide
Fig. 1. FTIR spectra of prepared samples.
2. Experimental 2.1. Materials and instruments RhB was obtained from Samchun Pure Chemical Co., Ltd. All the other reagents and chemicals used in our study were acquired from Merck. Fourier-transform infrared spectroscopy (Perkin-Elmer, Spectrum One), scanning electron microscopy (FEI Teneo), transmission electron microscopy (FEI Tecnai G2 Spirit TEM), X-ray diffraction measurement (Bruker D8 Discover), N2 adsorption-desorption (BELSORP-mini II), inductively coupled plasma mass spectroscopy (Dionex Corporation) and double beam UV–Vis spectrophotometer (Perkin-Elmer, Lambda 25) were applied for characterization of prepared materials.
Fig. 2. XRD patterns of prepared materials.
lined stainless steel autoclave and heated for 24 h at 120 °C in oven. After cooling down the autoclave, the resulting white sediment was centrifuged and washed with fresh DMF and methanol for several times. Finally the UiO-66 powder was dried over night at 70 °C.
2.2. Synthesis of the materials 2.2.1. Preparation of UiO-66 According to the procedure described in the literature [32], UiO-66 particles were synthesized. Briefly, 0.095 g of ZrCl4 (0.4 mmol) and the ligand precursor contains 0.067 g (0.4 mmol) of H2BDC were dissolved in 15 mL of DMF in a beaker separately, followed by sonicating for 15 min each beaker. Subsequently, Zr4+ solution was added to the H2BDC solution, followed by adding 6 mL of acetic acid and stirring for 60 min. After that, the mixed solution was transferred into a Teflon-
2.2.2. Preparation of TiO2/UiO-66 nanocomposite TiO2/UiO-66 nanocomposite was synthesized according to the reflux method. At first, 0.2 g of UiO-66 was added to 50 mL of ethanol and dispersed with ultrasonic for 20 min. Then, 1 mL of tetra-n-butylorthotitanat was added to the solution and sonicated for 20 min. After that, 2 mL of deionized water was added to the solution and after 2
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Fig. 3. SEM images of prepared samples: (a) Pure UiO-66, (b) Calcined TiO2/UiO-66 nanocomposite (TiO2/ZrO2) and (c, d) TEM images of synthesized UiO-66 crystal.
degradation. The effect of pH was also studied with adjusting initial pH by HCl (0.1 M) and NaOH (0.1 M) using a pH-meter.
stirring at room temperature for 30 min, the mixed solution was refluxed at 100 ℃ for 3 h. After centrifugation, the white powder was separated, washed with fresh ethanol, and this step was repeated three times, then the powder was dried at 60 ℃ in the oven. At last, porous TiO2/ZrO2 was obtained by calcining the Titania/MOF nanocomposites at 450 °C for 2 h.
3. Result and discussion 3.1. Characterization of materials
2.3. Photocatalytic dye degradation
The results of FTIR analysis proved the existence of the functional groups in the synthesized materials structures. As shown in Fig. 1, FTIR spectra of porous TiO2/ZrO2 has been compared with the parent UiO-66 and bare TiO2 nanoparticles. The broad band at 500-900 cm−1 was added to the spectrum that was assigned to the stretching of Ti–O bond and bending of O–Ti–O bond [33]. The IR spectra of calcined TiO2/UiO66 nanocomposite exhibits UiO-66 bands which corroborated the presence of MOF in nanocomposite structure and suggested that the structural characteristics of the MOF are retained after adding TiO2. Furthermore, the observed changes in comparison with the pure UiO-66 showed that there are interactions between two incorporators of the nanocomposite. The powder X-ray diffraction analysis was employed to identify the crystalline structures of the synthesized samples. Fig. 2 shows the XRD pattern of UiO-66 in the form of white powder which is in good agreement with other simulated patterns reported in the literatures [32,34,35] indicating the successful synthesis of the MOF. The diffraction peaks at 2θ = 7.38, 8.51, 14.08, 14.84, 15.42, 17.12, 18.58, 19.08, 21.04, 22.2, 24.12, 25.62, 28.16 and 29.86° correspond to the (111), (200), (311), (222), (400), (311), (420), (422), (511), (440), (531), (600), (622) and (444) planes, respectively. The XRD pattern of TiO2 nanoparticle showed that the diffraction peaks at 25° and 48° are
For investigating photocatalytic degradation of RhB in a batch reactor, 200 mL solution of dye (20 mg L−1) was prepared and certain amounts of porous TiO2/ZrO2 were added to the solution. In the first step, the suspension was completely stirred in the dark using a magnetic stirrer for 30 min in order to attain complete adsorption equilibrium. Then, the mixture was irradiated by a 100 W LED lamp under visiblelight on a magnetic stirrer for 3 h. Afterwards, 5 mL of sample was collected from the solution at an interval time of 30 min, during 3 h reaction time. Finally, samples were separated from photocatalyst by using a centrifuge at 6000 rpm and the concentration of dye remaining in solution measured by a spectrophotometer at the maximum adsorption wavelength of RhB (λmax = 554 nm). Finally, the removal efficiency were computed by the following equation:
Removal efficiency (%) =
C 0 − Ct × 100 C0
(1)
where C0 is the initial concentration of RhB and Ct is the dye concentration at each interval time. The experiments were repeated with different amount of photocatalyst and dye concentration to peruse these factors’ effects on RhB 3
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Fig. 4. Elemental mapping and EDS spectrum of synthesized TiO2/ZrO2 porous photocatalyst.
octahedral shape. The MOF particles have a uniform distribution size between 80–100 nm. TEM images of crystals and topological shape of UiO-66 particles confirm the bipyramidal shaped of the MOF (Fig.3c, d). As shown in Fig. 3b, the UiO-66 crystals were well composed or covered with the formed TiO2 particles. Also, owing to calcination at high temperature, the nanocatalyst particles were aggregated. In addition, the elemental analysis was performed after composition of UiO-66 and TiO2 and the results were illustrated in Fig. 4. The final
attributed to the anatase phase of Titania [36]. As can be seen from the XRD pattern of TiO2/UiO-66, it could be concluded that almost all specific peaks of TiO2 have been overlapped and covered by UiO-66 peaks. This claim can be clearly confirmed based on the anatase phase of TiO2 from JCPDS card No. 21-1272. In order to determine shape, particle size, and morphology of synthesized materials, scanning electron microscopy (SEM) was utilized. As can be observed from Fig. 3a, UiO-66 crystals possess monodisperse 4
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Fig. 5. UV–vis/DRS (left) and the estimated band gap (right) for synthesized materials.
The UV–vis adsorption analysis data was employed to specify and calculate the band gap. The band gap is determined by plotting the (αhν)2 versus hν which is known as the Tauc plot. As shown in Fig. 5, the Eg of the prior TiO2 and UiO-66 are estimated to be 3.26 and 4.05 eV, respectively. Moreover, the amount of Eg in 3.88 eV is found for TiO2/ZrO2 porous photocatalyst, greatly improves the photocatalytic ability of the catalyst in visible light which indicating the successful hybridization of UiO-66 and TiO2. N2 adsorption-desorption study was conducted to determine the BET surface area and pore volume of the as-prepared UiO-66 and TiO2/ ZrO2 porous photocatalyst (Fig. 6). It can be concluded that configuration of pores is mostly the microporous structure due to the fact that both synthesized Zr-MOF based particles match with the typical type I isotherms in IUPAC category for porous materials. UiO-66 and TiO2/ ZrO2 porous photocatalyst showed a large surface area of 1120 and 916 m2 g−1 and a total pore volume of 0.529 and 0.41 cm3 g−1, respectively. Since Titania has blocked the cavity widow, the specific surface area of porous TiO2/ZrO2 is lower than the pure UiO-66. According to the IUPAC classification of porous materials, the microporous adsorbent size range is the material with a mean pore size of < 2 nm [19,37]. The mean pore diameter of the pores for synthesized UiO-66 and TiO2/ZrO2 were measured 1.89 and 1.74 nm, respectively. Thus, both synthesized Zr-MOF based are microporous material.
Fig. 6. Nitrogen adsorption-desorption isotherms for the synthesized samples at 77 K.
compound of the calcined TiO2/UiO-66 nanocomposite includes the related pieces, which is in great agreement with the formation of MOF composite. As shown in the inset, EDS spectrum could be helpful in determining the weight percentages and the each elements’ quantity in MOF composite structure. Also, the visual overview of density has been provided by the elemental mapping which showed a uniform distribution of Zr, Ti, C, and O in the nanocomposite structure.
3.2. Photocatalytic activity 3.2.1. Evaluation of the photocatalysts performance To introduce calcined UiO-66/TiO2 heterostructures as good materials for efficiently removal of contaminants, the photocatalytic activity of TiO2/ZrO2 was compared with different samples for degradation of
Fig. 7. Photocatalytic curves (left) and the rate constant of photodegradation reaction (right) using different photocatalysts after 180 min of visible light irradiation. 5
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Fig. 8. Impact of involved parameters in photodegradation of RhB: (a) catalyst dosage (b) solution pH, and (c) RhB concentration. Table 1 Comparison of the Rhodamin B degradation between the porous TiO2/ZrO2 nanocomposite and other reported photocatalysts. Catalyst
RhB Concentration
Amount (g/L)
Source
Time (min)
Degradation (%)
Ref.
UiO-66 BiOBr/UiO-66 Carbonized CuBTC Zn3(PO4)2/BiPO4 NiFe2O4/MIL-53(Fe) ZnO-graphene-TiO2 ZnFe2O4@g-C3N4 Ce/Mo-V4O9 TiO2/ZrO2
0.03 mM 0.03 mM 25 mg/L 0.01 mM 0.03 mM 0.01 mM 10 mg/L 0.01 mM 20 mg/L
5 5 11 1 0.2 1 0.2 0.1 0.8
UV light UV light UV light UV light Visible light Visible light Visible light Visible light Visible light
15 15 5 60 180 180 30 180 180
7 95 98 99 95 92 96 88 99
[39] [39] [40] [41] [42] [43] [44] [45] This work
efficient usage of energy and enhancing the photocatalytic efficiency [38]. Visible light can be utilized by TiO2 particles, but since the recombination rate of photo-generated electrons and holes is high, the photocatalytic effect obtained relatively low. The photocatalytic effect of the mechanical mixture of prior UiO-66 and TiO2 (1:1) was similar to UiO-66 with a slightly higher performance. After calcination of TiO2/ UiO-66 nanocomposite, the obtained porous TiO2/ZrO2 showed excellent photocatalytic performance by 95% degradation of RhB. In fact, the photocatalytic efficiency increased since the recombination of photo-generated electrons and holes could be reduced by the formed heterojunctions. The photodegradation data were well fitted with a fist-order kinetic. The photocatalytic decolorization rate constant (k) for RhB removal were shown in Fig. 7. As can be seen, the values of k for each one includes: Porous TiO2/ZrO2 (0.013 min−1) > physical mixture of
Rhodamin B (RhB) at the same operational conditions. Fig. 7 shows the photocatalytic activities of pure UiO-66, TiO2 particles, mechanical mixture of prior UiO-66 and TiO2 and porous TiO2/ZrO2 under visible light irradiation. In order to survey the photocatalytic activity of the catalyst apart from removal of pollutant through adsorption-desorption, the mixture was stirred for 60 min in the dark and the initial concentration (C0) of the RhB was considered the adsorption-desorption equilibration. The RhB solution (200 mL) without catalyst in the presence of 40 μL H2O2 degrade for 7% after 180 min, which confirms the stability of RhB against visible light. After being stirred for 60 min in the dark, the pure UiO-66 showed the best adsorption ability, which is due to its high surface area. This feature could helped to attract the dye molecules and excellent photocatalytic performance as well. MOF structure facilitates the removal of dye in two ways: first, dye molecules could easily be adsorbed on the catalyst surface, and second, the photocatalyst could more readily adsorb photons which results in a more 6
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because of increment in electrostatic attractive forces between contaminant and surface of catalyst. According to the results, pH 9 was selected as the optimum solution pH. The effect of dye concentration was surveyed by differing the initial concentration of RhB. According to Fig. 8c, the results confirmed that by increasing the dye concentration, the dye degradation decreased. This is because of the fact that less LED light irradiation passes through the solution and thus, the efficiency of photodegradation process lowers in highly concentrated dye solution. The removal rate decreased from 95% to 46% as the concentration of RhB changes from 20 ppm to 40 ppm. In order to evaluate the Rhodamin B degradation by the synthesized porous TiO2/ZrO2 nanocatalyst in oxidation process, the obtained results from this study was compared with other reported photocatalysts in literature, as illustrated in Table 1 [39–45]. 3.3. Photodegradation mechanism The molecular structure of dyes are big enough to be mostly absorbed on the outer surface of catalyst. Therefore, the proposed mechanism was dual-functional mechanism consisting of two steps, as depicted in Fig. 9. The high electrostatic attraction among the adsorbents and molecules of dyes is the reason why the adsorption efficiency was high. Cationic dyes are most likely to be absorbed by the exposed −COO− on the outer surface of catalyst [46]. Then, by stimulating with light irradiation, photo-excited electrons would be transferred into the conduction band of TiO2 through the quantum tunneling effect [47], then the free radicals of OH• is produced through reaction of the h+ with H2O or free radicals of O2•- can be formed from the reaction between e− with O2 which is adsorbed on the surface. These reactions directly participate in degradation of RhB, which are summarized as followed:
Fig. 9. The proposed RhB photocatalytic degradation mechanism using TiO2/ ZrO2 photocatalyst.
prior UiO-66 and TiO2 (0.0036 min−1) > UiO-66 (0.0032 min−1) > TiO2 (0.0022 min−1) > bare LED light (0.0005 min−1). The results further indicate that an appreciable calcination of Titania/MOF nanocomposite enhance photocatalytic performance. 3.2.2. Effect of operational parameters In our work, through study was done on impact of three most effective parameters on remediation of RhB molecules (dosage of catalyst, pH, and RhB concentration). In this part, we differed the concentration of TiO2/ZrO2 in order to investigate the effect of photocatalyst dosage on the photo-degradation process. The experiment was done using different amount of calcined Titania/MOF (0.2, 0.4, 0.6 and 0.8 mg L−1). As depicted in Fig. 8a, the results showed that by increasing the amount of photocatalyst existing in the solution, the dye degradation increased. Clearly, with the higher amount of photocatalyst, the rate of photodegradation reaction speeds up but this happens until a maximum dosage in which by increasing the dosage of photocatalyst, the removal rate diminishes. This is due to the fact that the photocatalyst barricades the LED light irradiation and limits the reaction of oxidation. Fig. 8b shows that the photodegradation of RhB was effected by changing solution pH at the optimum concentration of RhB and TiO2/ ZrO2 dose. Zeta potential analysis shown the surface charge of the catalyst became positive at acidic region and pushed away cationic molecules of RhB. However, surface adsorption increased at higher pH
RhB + hν → RhB∗ → RhB• + + e− TiO2 + hν → TiO2 (h+ + e−) UiO − 66 + e− → Zr 3 + − UiO − 66 Zr 3 + − UiO − 66 + O2 → Zr 4 + − UiO − 66 + O2• − O2• −/ OH • + RhB /RhB• + → degredation products h+ + RhB /RhB• + → degredation products
3.4. Stability and reusability The stability and recyclability of catalysts are important parameters for their industrial and large-scale application. For this purpose, utilized nanoporous photocatalyst particles were collected after each degradation process and washed with 10 mL ethanol/water solution for 10 h
Fig. 10. (a) Photocatalytic performance of the recycled catalyst (0.8 g/L catalyst dosage, 20 mg/L RhB concentration and initial pH=, process time = 180 min), and (b) SEM image of porous TiO2/ZrO2 after fifth cycle photocatalytic process. 7
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then dehumidified at 70 °C in oven then utilized in the subsequent run. According to Fig. 10a, degradation efficiency of TiO2/ZrO2 slightly decreased to 90% of its initial state after four runs and can considered as a recoverable and economical catalyst for degradation of RhB. In addition, the morphology of porous TiO2/ZrO2 was investigated after four-reuse cycle by SEM analysis (Fig. 10b). The image shows that the structure of photocatalyst does not significantly change after five times recycling, indicating its high stability in water. In order to determine the stability of the TiO2/ZrO2 composite, the quantity of Zr and Ti elements was measured in the degradation liquid. The photo-corrosion process of the composite is the reason for presence of Zr and Ti elements in the degradation liquid. Conducting the degradation experiment using inductively coupled plasma (ICP) showed that the detected amount of Zr element in the both samples was zero which depicts excellent stability of UiO-66. Almost, less than 1% (0.41 mg/L) of Ti element from a certain concentration of TiO2/ZrO2 (60 mg/L) was found residual in the degradation liquid. So, authors assert the prepared porous nanocomposite is a stable material in aqueous medium.
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4. Conclusion In summary, the novel porous TiO2/ZrO2 was synthesized by calcination of Titania/UiO-66 nanocomposite, which was utilized as photocatalyst for mineralization of RhB under visible light irradiation. FTIR, SEM/EDS, XRD, TEM, BET, UV-DRS and ICP analysis confirmed the UiO-66, TiO2 and TiO2/ZrO2 components. The photocatalytic activity of porous TiO2/ZrO2 photocatalyst was higher than the mixture of prior UiO-66 and TiO2. This was because of the fact that the UiO-66 and TiO2 were just physically mixed and there was no heterojunction formed between them. The fist-order kinetic had the best fit with degradation data and the porous TiO2/ZrO2 photocatalyst showed the highest rate constant of 0.013 min−1. The results demonstrated that the TiO2/ZrO2 composite possessed high efficiency (90%) after being cycled for four times with a simple regeneration process. This work introduces a new photocatalyst derived from MOFs as a highly efficient catalysts in environmental applications especially in removing organic dyes from wastewater and provides detail of the design and synthesis of these porous materials. Acknowledgement The authors would like to acknowledge Department of Chemical and Petroleum Engineering, Sharif University of Technology for the financial support during this research. References [1] Y. Fu, T. Viraraghavan, Fungal decolorization of dye wastewaters: a review, Bioresour. Technol. 79 (2001) 251–262. [2] C. Pearce, J. Lloyd, J. Guthrie, The removal of colour from textile wastewater using whole bacterial cells: a review, Dyes Pigm. 58 (2003) 179–196. [3] G. Crini, Non-conventional low-cost adsorbents for dye removal: a review, Bioresour. Technol. 97 (2006) 1061–1085. [4] W. Feng, D. Nansheng, Z. Yuegang, Discoloration of dye solutions induced by solar photolysis of ferrioxalate in aqueous solutions, Chemosphere 39 (1999) 2079–2085. [5] D. Mehta, S. Mazumdar, S. Singh, Magnetic adsorbents for the treatment of water/ wastewater—a review, J. Water Process Eng. 7 (2015) 244–265. [6] Y.M. Slokar, A.M. Le Marechal, Methods of decoloration of textile wastewaters, Dyes Pigm. 37 (1998) 335–356. [7] C. Fernández, M.S. Larrechi, M.P. Callao, An analytical overview of processes for removing organic dyes from wastewater effluents, TrAC, Trends Analyt. Chem. 29 (2010) 1202–1211. [8] Y. Deng, R. Zhao, Advanced oxidation processes (AOPs) in wastewater treatment, Curr. Pollut. Rep. 1 (2015) 167–176. [9] K. Ayoub, E.D. Van Hullebusch, M. Cassir, A. Bermond, Application of advanced oxidation processes for TNT removal: a review, J. Hazard. Mater. 178 (2010) 10–28. [10] N.H. Ince, D.T. Gönenç, Treatability of a textile azo dye by UV/H2O2, Environ. Technol. 18 (1997) 179–185. [11] S.-F. Kang, C.-H. Liao, S.-T. Po, Decolorization of textile wastewater by photoFenton oxidation technology, Chemosphere 41 (2000) 1287–1294.
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[40] Z. Hasan, D.-W. Cho, G.J. Islam, H. Song, Catalytic decoloration of commercial azo dyes by copper-carbon composites derived from metal organic frameworks, J. Alloys Compd. 689 (2016) 625–631. [41] Y. Naciri, A. Chennah, C. Jaramillo-Páez, J.A. Navío, B. Bakiz, A. Taoufyq, M. Ezahri, S. Villain, F. Guinneton, A. Benlhachem, Preparation, characterization and photocatalytic degradation of Rhodamine B dye over a novel Zn3(PO4)2/BiPO4 catalyst, J. Environ. Chem. Eng. (2019) 103075. [42] V.H. Nguyen, L.G. Bach, Q.T.P. Bui, T.D. Nguyen, D.-V.N. Vo, H.T. Vu, S.T. Do, Composite photocatalysts containing MIL-53 (Fe) as a heterogeneous photo-Fenton catalyst for the decolorization of rhodamine B under visible light irradiation, J. Environ. Chem. Eng. 6 (2018) 7434–7441. [43] P. Nuengmatcha, S. Chanthai, R. Mahachai, W.-C. Oh, Visible light-driven photocatalytic degradation of rhodamine B and industrial dyes (texbrite BAC-L and
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texbrite NFW-L) by ZnO-graphene-TiO2 composite, J. Environ. Chem. Eng. 4 (2016) 2170–2177. S. Borthakur, L. Saikia, ZnFe2O4@g-C3N4 nanocomposites: an efficient catalyst for fenton-like photodegradation of environmentally pollutant rhodamine B, J. Environ. Chem. Eng. (2019) 103035. R. Arunadevi, B. Kavitha, M. Rajarajan, A. Suganthi, Synthesis of Ce/Mo-V4O9 nanoparticles with superior visible light photocatalytic activity for Rhodamine-B degradation, J. Environ. Chem. Eng. 6 (2018) 3349–3357. Q. Chen, Q. He, M. Lv, Y. Xu, H. Yang, X. Liu, F. Wei, Selective adsorption of cationic dyes by UiO-66-NH2, Appl. Surf. Sci. 327 (2015) 77–85. Y. Li, A. Pang, C. Wang, M. Wei, Metal–organic frameworks: promising materials for improving the open circuit voltage of dye-sensitized solar cells, J. Mater. Chem. 21 (2011) 17259–17264.
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Synthesis of porous TiO2/ZrO2 photocatalyst derived from zirconium metal organic framework for degradation of organic pollutants under visible light irradiation ⁎
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Jafar Abdi , Maede Yahyanezhad, Sahar Sakhaie, Manouchehr Vossoughi , Iran Alemzadeh Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Metal-organic frameworks Photocatalyst Porous TiO2/ZrO2 Synthesis Wastewater treatment
Zirconium based metal-organic frameworks (Zr-MOFs) are promising candidates for photocatalytic wastewater treatment due to their excellent properties such as high chemical and thermal stability and high photodegradation ability. Herein, we report a novel porous TiO2/ZrO2 photocatalyst derived from UiO-66 and Titania hybrids. UiO-66 nanoparticles was synthesized through solvothermal method and utilized as catalyst support to grow TiO2 particles on its surface. The prepared Titania/MOF nanocomposite was calcined to obtain porous TiO2/ZrO2 photocatalyst for degradation of organic pollutants from colored wastewater under LED visible light. The prepared materials were fully characterized with FTIR, XRD, SEM/EDS, TEM, BET, UV-DRS and ICP analysis. The results showed that the developed TiO2/ZrO2 enhanced photodegradation ability of Rhodamin B (RhB) in comparison with the mixture of prior UiO-66 and TiO2 and was found to affect the photocatalytic activity by increasing the adsorption of photons in visible region and enhanced the transfer and separation of produced charge. The decolorization kinetics followed first-order kinetic model. In addition, after four times recycling, the regenerated nanocomposite still showed high stability and photodegradation ability (90%).
1. Introduction In recent years, water pollution and the crisis of water shortage, are the most important problems that humans have been faced. A great amount of water pollution comes from chemical industries such as textile or paper industry [1]. Wastewater treatment of textile industry is the most difficult one among others since textile industry utilizes more than 7 × 105 tones dyes annually and 2% of the amount of dyes, which has been used, is discharged into the effluent and 10% is lost during the dyeing processes [2,3]. Removal of dyes form colored wastewaters is more essential because a small amount of dyes in wastewater is harmful for humans’ environment and ecosystem [4]. Furthermore, in comparison to other contaminants, dyes are more resistant to conventional methods of wastewater treatment. Several methods such as physical, biological and chemical processes are employed for water purification and removal of different contaminants from wastewaters [5,6]. Among different treatment methods, chemical treatments especially advanced oxidation processes (AOPs) counts as one of the most effective methods in dye removal from wastewater [7,8]. Some advantages of AOPs are that these processes
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operate at or near ambient temperature and pressure and convert approximately all the organic compounds contaminating effluents into less hazardous products [9]. Processes using H2O2 or UV irradiation [10], Fenton and photo-Fenton catalytic reactions as some examples of AOPs [11], are efficacious methods based on the generation of hydroxyl radicals which oxidizes a wide range of organic pollutants in wastewater non-selectively. Using photocatalytic degradation processes under UV/visible irradiation is a newborn and green method in wastewater treatment especially in the field of dye degradation. Several photocatalysts such as TiO2, ZrO2, ZnO, etc. have been used for destroying pollutants [12–15]. Among different photocatalysts, TiO2 has attracted a great attention due to the fact that it is not toxic and harmful for the environment and has a low-cost synthesis. Besides, there are many studies on TiO2 among other photocatalyst; Although, the band gap of TiO2 is relatively wide (3.2 eV) which makes it not effective enough under visible light [15]. However, whether semiconductors combined with some other porous materials may enhance the photocatalytic performance and make the photocatalytic processes more sufficient. One of the new emerged compounds named metal organic frameworks (MOFs) have attracted
Corresponding author. Corresponding author. E-mail addresses: [email protected], [email protected] (J. Abdi), [email protected] (M. Vossoughi).
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https://doi.org/10.1016/j.jece.2019.103096 Received 19 March 2019; Received in revised form 11 April 2019; Accepted 13 April 2019 Available online 16 April 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
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Nomenclature UV LED FTIR SEM TEM XRD
BET EDS DRS DMF BDC HCl NaOH
Ultra violet Light-emitting diode Fourier transform infrared Scanning electron microscopy Transmission electron microscopy X-ray diffraction
significant attention as a photocatalyst [16,17]. MOFs as organic-inorganic hybrid materials are a new kind of porous compound which have been surveyed in several fields such as catalysis [18–20], gas storage [21], sensors [22], chemical separation [23], adsorption [16], etc. These materials have attracted great attention because of their high surface area, high porosity, and low density [24]. Several studies have been conducted on MOFs, results of which have shown that these structures can act as a photocatalyst under UV or visible irradiation in Rhodamin B (RhB) degradation process [25–29]. As one of Zr-based MOFs, UiO-66, possesses high thermal and mechanical stability [30]. Nevertheless, the band gap of UiO-66 is about 3.6 eV which limit its optical adsorption in the visible light region. To achieve this, a narrow band gap semiconductors coupling formed a heterojunction method is an ideal choice. The heterojunction can form an inner electric field to facilitate the transfer/separation of photogenerated electron/hole and inhibit the recombination of electrons and hole so as to enhance the photocatalytic activity [31]. So, it is rational to expect that by hybridizing UiO-66 with another excellent semiconductors, a more stable and efficacious MOF composite with photocatalytic activity would be attained. In this study, we are reporting porous TiO2/ZrO2 derived Titania/ MOF nanocomposite as a photocatalyst for degradation of RhB under LED visible irradiations for the first time. The functional group, crystalline structure, morphology and photocatalytic activity of prepared material was investigated by FTIR, XRD, SEM/EDS, TEM, BET, UV-DRS, and ICP analysis. The results showed that incorporation of Zr-MOF and TiO2 increased photodegradation ability of RhB. Effects of different factors such as initial solution pH, amounts of photocatalyst and dye concentration were investigated in dye degradation process.
Brunauer-Emmett-Teller Energy-dispersive X-ray spectrometry Diffuse reflectance spectroscopy Dimethylformamide Benzene-dicarboxylic acid Hydrochloric acid Sodium hydroxide
Fig. 1. FTIR spectra of prepared samples.
2. Experimental 2.1. Materials and instruments RhB was obtained from Samchun Pure Chemical Co., Ltd. All the other reagents and chemicals used in our study were acquired from Merck. Fourier-transform infrared spectroscopy (Perkin-Elmer, Spectrum One), scanning electron microscopy (FEI Teneo), transmission electron microscopy (FEI Tecnai G2 Spirit TEM), X-ray diffraction measurement (Bruker D8 Discover), N2 adsorption-desorption (BELSORP-mini II), inductively coupled plasma mass spectroscopy (Dionex Corporation) and double beam UV–Vis spectrophotometer (Perkin-Elmer, Lambda 25) were applied for characterization of prepared materials.
Fig. 2. XRD patterns of prepared materials.
lined stainless steel autoclave and heated for 24 h at 120 °C in oven. After cooling down the autoclave, the resulting white sediment was centrifuged and washed with fresh DMF and methanol for several times. Finally the UiO-66 powder was dried over night at 70 °C.
2.2. Synthesis of the materials 2.2.1. Preparation of UiO-66 According to the procedure described in the literature [32], UiO-66 particles were synthesized. Briefly, 0.095 g of ZrCl4 (0.4 mmol) and the ligand precursor contains 0.067 g (0.4 mmol) of H2BDC were dissolved in 15 mL of DMF in a beaker separately, followed by sonicating for 15 min each beaker. Subsequently, Zr4+ solution was added to the H2BDC solution, followed by adding 6 mL of acetic acid and stirring for 60 min. After that, the mixed solution was transferred into a Teflon-
2.2.2. Preparation of TiO2/UiO-66 nanocomposite TiO2/UiO-66 nanocomposite was synthesized according to the reflux method. At first, 0.2 g of UiO-66 was added to 50 mL of ethanol and dispersed with ultrasonic for 20 min. Then, 1 mL of tetra-n-butylorthotitanat was added to the solution and sonicated for 20 min. After that, 2 mL of deionized water was added to the solution and after 2
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Fig. 3. SEM images of prepared samples: (a) Pure UiO-66, (b) Calcined TiO2/UiO-66 nanocomposite (TiO2/ZrO2) and (c, d) TEM images of synthesized UiO-66 crystal.
degradation. The effect of pH was also studied with adjusting initial pH by HCl (0.1 M) and NaOH (0.1 M) using a pH-meter.
stirring at room temperature for 30 min, the mixed solution was refluxed at 100 ℃ for 3 h. After centrifugation, the white powder was separated, washed with fresh ethanol, and this step was repeated three times, then the powder was dried at 60 ℃ in the oven. At last, porous TiO2/ZrO2 was obtained by calcining the Titania/MOF nanocomposites at 450 °C for 2 h.
3. Result and discussion 3.1. Characterization of materials
2.3. Photocatalytic dye degradation
The results of FTIR analysis proved the existence of the functional groups in the synthesized materials structures. As shown in Fig. 1, FTIR spectra of porous TiO2/ZrO2 has been compared with the parent UiO-66 and bare TiO2 nanoparticles. The broad band at 500-900 cm−1 was added to the spectrum that was assigned to the stretching of Ti–O bond and bending of O–Ti–O bond [33]. The IR spectra of calcined TiO2/UiO66 nanocomposite exhibits UiO-66 bands which corroborated the presence of MOF in nanocomposite structure and suggested that the structural characteristics of the MOF are retained after adding TiO2. Furthermore, the observed changes in comparison with the pure UiO-66 showed that there are interactions between two incorporators of the nanocomposite. The powder X-ray diffraction analysis was employed to identify the crystalline structures of the synthesized samples. Fig. 2 shows the XRD pattern of UiO-66 in the form of white powder which is in good agreement with other simulated patterns reported in the literatures [32,34,35] indicating the successful synthesis of the MOF. The diffraction peaks at 2θ = 7.38, 8.51, 14.08, 14.84, 15.42, 17.12, 18.58, 19.08, 21.04, 22.2, 24.12, 25.62, 28.16 and 29.86° correspond to the (111), (200), (311), (222), (400), (311), (420), (422), (511), (440), (531), (600), (622) and (444) planes, respectively. The XRD pattern of TiO2 nanoparticle showed that the diffraction peaks at 25° and 48° are
For investigating photocatalytic degradation of RhB in a batch reactor, 200 mL solution of dye (20 mg L−1) was prepared and certain amounts of porous TiO2/ZrO2 were added to the solution. In the first step, the suspension was completely stirred in the dark using a magnetic stirrer for 30 min in order to attain complete adsorption equilibrium. Then, the mixture was irradiated by a 100 W LED lamp under visiblelight on a magnetic stirrer for 3 h. Afterwards, 5 mL of sample was collected from the solution at an interval time of 30 min, during 3 h reaction time. Finally, samples were separated from photocatalyst by using a centrifuge at 6000 rpm and the concentration of dye remaining in solution measured by a spectrophotometer at the maximum adsorption wavelength of RhB (λmax = 554 nm). Finally, the removal efficiency were computed by the following equation:
Removal efficiency (%) =
C 0 − Ct × 100 C0
(1)
where C0 is the initial concentration of RhB and Ct is the dye concentration at each interval time. The experiments were repeated with different amount of photocatalyst and dye concentration to peruse these factors’ effects on RhB 3
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Fig. 4. Elemental mapping and EDS spectrum of synthesized TiO2/ZrO2 porous photocatalyst.
octahedral shape. The MOF particles have a uniform distribution size between 80–100 nm. TEM images of crystals and topological shape of UiO-66 particles confirm the bipyramidal shaped of the MOF (Fig.3c, d). As shown in Fig. 3b, the UiO-66 crystals were well composed or covered with the formed TiO2 particles. Also, owing to calcination at high temperature, the nanocatalyst particles were aggregated. In addition, the elemental analysis was performed after composition of UiO-66 and TiO2 and the results were illustrated in Fig. 4. The final
attributed to the anatase phase of Titania [36]. As can be seen from the XRD pattern of TiO2/UiO-66, it could be concluded that almost all specific peaks of TiO2 have been overlapped and covered by UiO-66 peaks. This claim can be clearly confirmed based on the anatase phase of TiO2 from JCPDS card No. 21-1272. In order to determine shape, particle size, and morphology of synthesized materials, scanning electron microscopy (SEM) was utilized. As can be observed from Fig. 3a, UiO-66 crystals possess monodisperse 4
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Fig. 5. UV–vis/DRS (left) and the estimated band gap (right) for synthesized materials.
The UV–vis adsorption analysis data was employed to specify and calculate the band gap. The band gap is determined by plotting the (αhν)2 versus hν which is known as the Tauc plot. As shown in Fig. 5, the Eg of the prior TiO2 and UiO-66 are estimated to be 3.26 and 4.05 eV, respectively. Moreover, the amount of Eg in 3.88 eV is found for TiO2/ZrO2 porous photocatalyst, greatly improves the photocatalytic ability of the catalyst in visible light which indicating the successful hybridization of UiO-66 and TiO2. N2 adsorption-desorption study was conducted to determine the BET surface area and pore volume of the as-prepared UiO-66 and TiO2/ ZrO2 porous photocatalyst (Fig. 6). It can be concluded that configuration of pores is mostly the microporous structure due to the fact that both synthesized Zr-MOF based particles match with the typical type I isotherms in IUPAC category for porous materials. UiO-66 and TiO2/ ZrO2 porous photocatalyst showed a large surface area of 1120 and 916 m2 g−1 and a total pore volume of 0.529 and 0.41 cm3 g−1, respectively. Since Titania has blocked the cavity widow, the specific surface area of porous TiO2/ZrO2 is lower than the pure UiO-66. According to the IUPAC classification of porous materials, the microporous adsorbent size range is the material with a mean pore size of < 2 nm [19,37]. The mean pore diameter of the pores for synthesized UiO-66 and TiO2/ZrO2 were measured 1.89 and 1.74 nm, respectively. Thus, both synthesized Zr-MOF based are microporous material.
Fig. 6. Nitrogen adsorption-desorption isotherms for the synthesized samples at 77 K.
compound of the calcined TiO2/UiO-66 nanocomposite includes the related pieces, which is in great agreement with the formation of MOF composite. As shown in the inset, EDS spectrum could be helpful in determining the weight percentages and the each elements’ quantity in MOF composite structure. Also, the visual overview of density has been provided by the elemental mapping which showed a uniform distribution of Zr, Ti, C, and O in the nanocomposite structure.
3.2. Photocatalytic activity 3.2.1. Evaluation of the photocatalysts performance To introduce calcined UiO-66/TiO2 heterostructures as good materials for efficiently removal of contaminants, the photocatalytic activity of TiO2/ZrO2 was compared with different samples for degradation of
Fig. 7. Photocatalytic curves (left) and the rate constant of photodegradation reaction (right) using different photocatalysts after 180 min of visible light irradiation. 5
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Fig. 8. Impact of involved parameters in photodegradation of RhB: (a) catalyst dosage (b) solution pH, and (c) RhB concentration. Table 1 Comparison of the Rhodamin B degradation between the porous TiO2/ZrO2 nanocomposite and other reported photocatalysts. Catalyst
RhB Concentration
Amount (g/L)
Source
Time (min)
Degradation (%)
Ref.
UiO-66 BiOBr/UiO-66 Carbonized CuBTC Zn3(PO4)2/BiPO4 NiFe2O4/MIL-53(Fe) ZnO-graphene-TiO2 ZnFe2O4@g-C3N4 Ce/Mo-V4O9 TiO2/ZrO2
0.03 mM 0.03 mM 25 mg/L 0.01 mM 0.03 mM 0.01 mM 10 mg/L 0.01 mM 20 mg/L
5 5 11 1 0.2 1 0.2 0.1 0.8
UV light UV light UV light UV light Visible light Visible light Visible light Visible light Visible light
15 15 5 60 180 180 30 180 180
7 95 98 99 95 92 96 88 99
[39] [39] [40] [41] [42] [43] [44] [45] This work
efficient usage of energy and enhancing the photocatalytic efficiency [38]. Visible light can be utilized by TiO2 particles, but since the recombination rate of photo-generated electrons and holes is high, the photocatalytic effect obtained relatively low. The photocatalytic effect of the mechanical mixture of prior UiO-66 and TiO2 (1:1) was similar to UiO-66 with a slightly higher performance. After calcination of TiO2/ UiO-66 nanocomposite, the obtained porous TiO2/ZrO2 showed excellent photocatalytic performance by 95% degradation of RhB. In fact, the photocatalytic efficiency increased since the recombination of photo-generated electrons and holes could be reduced by the formed heterojunctions. The photodegradation data were well fitted with a fist-order kinetic. The photocatalytic decolorization rate constant (k) for RhB removal were shown in Fig. 7. As can be seen, the values of k for each one includes: Porous TiO2/ZrO2 (0.013 min−1) > physical mixture of
Rhodamin B (RhB) at the same operational conditions. Fig. 7 shows the photocatalytic activities of pure UiO-66, TiO2 particles, mechanical mixture of prior UiO-66 and TiO2 and porous TiO2/ZrO2 under visible light irradiation. In order to survey the photocatalytic activity of the catalyst apart from removal of pollutant through adsorption-desorption, the mixture was stirred for 60 min in the dark and the initial concentration (C0) of the RhB was considered the adsorption-desorption equilibration. The RhB solution (200 mL) without catalyst in the presence of 40 μL H2O2 degrade for 7% after 180 min, which confirms the stability of RhB against visible light. After being stirred for 60 min in the dark, the pure UiO-66 showed the best adsorption ability, which is due to its high surface area. This feature could helped to attract the dye molecules and excellent photocatalytic performance as well. MOF structure facilitates the removal of dye in two ways: first, dye molecules could easily be adsorbed on the catalyst surface, and second, the photocatalyst could more readily adsorb photons which results in a more 6
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because of increment in electrostatic attractive forces between contaminant and surface of catalyst. According to the results, pH 9 was selected as the optimum solution pH. The effect of dye concentration was surveyed by differing the initial concentration of RhB. According to Fig. 8c, the results confirmed that by increasing the dye concentration, the dye degradation decreased. This is because of the fact that less LED light irradiation passes through the solution and thus, the efficiency of photodegradation process lowers in highly concentrated dye solution. The removal rate decreased from 95% to 46% as the concentration of RhB changes from 20 ppm to 40 ppm. In order to evaluate the Rhodamin B degradation by the synthesized porous TiO2/ZrO2 nanocatalyst in oxidation process, the obtained results from this study was compared with other reported photocatalysts in literature, as illustrated in Table 1 [39–45]. 3.3. Photodegradation mechanism The molecular structure of dyes are big enough to be mostly absorbed on the outer surface of catalyst. Therefore, the proposed mechanism was dual-functional mechanism consisting of two steps, as depicted in Fig. 9. The high electrostatic attraction among the adsorbents and molecules of dyes is the reason why the adsorption efficiency was high. Cationic dyes are most likely to be absorbed by the exposed −COO− on the outer surface of catalyst [46]. Then, by stimulating with light irradiation, photo-excited electrons would be transferred into the conduction band of TiO2 through the quantum tunneling effect [47], then the free radicals of OH• is produced through reaction of the h+ with H2O or free radicals of O2•- can be formed from the reaction between e− with O2 which is adsorbed on the surface. These reactions directly participate in degradation of RhB, which are summarized as followed:
Fig. 9. The proposed RhB photocatalytic degradation mechanism using TiO2/ ZrO2 photocatalyst.
prior UiO-66 and TiO2 (0.0036 min−1) > UiO-66 (0.0032 min−1) > TiO2 (0.0022 min−1) > bare LED light (0.0005 min−1). The results further indicate that an appreciable calcination of Titania/MOF nanocomposite enhance photocatalytic performance. 3.2.2. Effect of operational parameters In our work, through study was done on impact of three most effective parameters on remediation of RhB molecules (dosage of catalyst, pH, and RhB concentration). In this part, we differed the concentration of TiO2/ZrO2 in order to investigate the effect of photocatalyst dosage on the photo-degradation process. The experiment was done using different amount of calcined Titania/MOF (0.2, 0.4, 0.6 and 0.8 mg L−1). As depicted in Fig. 8a, the results showed that by increasing the amount of photocatalyst existing in the solution, the dye degradation increased. Clearly, with the higher amount of photocatalyst, the rate of photodegradation reaction speeds up but this happens until a maximum dosage in which by increasing the dosage of photocatalyst, the removal rate diminishes. This is due to the fact that the photocatalyst barricades the LED light irradiation and limits the reaction of oxidation. Fig. 8b shows that the photodegradation of RhB was effected by changing solution pH at the optimum concentration of RhB and TiO2/ ZrO2 dose. Zeta potential analysis shown the surface charge of the catalyst became positive at acidic region and pushed away cationic molecules of RhB. However, surface adsorption increased at higher pH
RhB + hν → RhB∗ → RhB• + + e− TiO2 + hν → TiO2 (h+ + e−) UiO − 66 + e− → Zr 3 + − UiO − 66 Zr 3 + − UiO − 66 + O2 → Zr 4 + − UiO − 66 + O2• − O2• −/ OH • + RhB /RhB• + → degredation products h+ + RhB /RhB• + → degredation products
3.4. Stability and reusability The stability and recyclability of catalysts are important parameters for their industrial and large-scale application. For this purpose, utilized nanoporous photocatalyst particles were collected after each degradation process and washed with 10 mL ethanol/water solution for 10 h
Fig. 10. (a) Photocatalytic performance of the recycled catalyst (0.8 g/L catalyst dosage, 20 mg/L RhB concentration and initial pH=, process time = 180 min), and (b) SEM image of porous TiO2/ZrO2 after fifth cycle photocatalytic process. 7
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then dehumidified at 70 °C in oven then utilized in the subsequent run. According to Fig. 10a, degradation efficiency of TiO2/ZrO2 slightly decreased to 90% of its initial state after four runs and can considered as a recoverable and economical catalyst for degradation of RhB. In addition, the morphology of porous TiO2/ZrO2 was investigated after four-reuse cycle by SEM analysis (Fig. 10b). The image shows that the structure of photocatalyst does not significantly change after five times recycling, indicating its high stability in water. In order to determine the stability of the TiO2/ZrO2 composite, the quantity of Zr and Ti elements was measured in the degradation liquid. The photo-corrosion process of the composite is the reason for presence of Zr and Ti elements in the degradation liquid. Conducting the degradation experiment using inductively coupled plasma (ICP) showed that the detected amount of Zr element in the both samples was zero which depicts excellent stability of UiO-66. Almost, less than 1% (0.41 mg/L) of Ti element from a certain concentration of TiO2/ZrO2 (60 mg/L) was found residual in the degradation liquid. So, authors assert the prepared porous nanocomposite is a stable material in aqueous medium.
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4. Conclusion In summary, the novel porous TiO2/ZrO2 was synthesized by calcination of Titania/UiO-66 nanocomposite, which was utilized as photocatalyst for mineralization of RhB under visible light irradiation. FTIR, SEM/EDS, XRD, TEM, BET, UV-DRS and ICP analysis confirmed the UiO-66, TiO2 and TiO2/ZrO2 components. The photocatalytic activity of porous TiO2/ZrO2 photocatalyst was higher than the mixture of prior UiO-66 and TiO2. This was because of the fact that the UiO-66 and TiO2 were just physically mixed and there was no heterojunction formed between them. The fist-order kinetic had the best fit with degradation data and the porous TiO2/ZrO2 photocatalyst showed the highest rate constant of 0.013 min−1. The results demonstrated that the TiO2/ZrO2 composite possessed high efficiency (90%) after being cycled for four times with a simple regeneration process. This work introduces a new photocatalyst derived from MOFs as a highly efficient catalysts in environmental applications especially in removing organic dyes from wastewater and provides detail of the design and synthesis of these porous materials. Acknowledgement The authors would like to acknowledge Department of Chemical and Petroleum Engineering, Sharif University of Technology for the financial support during this research. References [1] Y. Fu, T. Viraraghavan, Fungal decolorization of dye wastewaters: a review, Bioresour. Technol. 79 (2001) 251–262. [2] C. Pearce, J. Lloyd, J. Guthrie, The removal of colour from textile wastewater using whole bacterial cells: a review, Dyes Pigm. 58 (2003) 179–196. [3] G. Crini, Non-conventional low-cost adsorbents for dye removal: a review, Bioresour. Technol. 97 (2006) 1061–1085. [4] W. Feng, D. Nansheng, Z. Yuegang, Discoloration of dye solutions induced by solar photolysis of ferrioxalate in aqueous solutions, Chemosphere 39 (1999) 2079–2085. [5] D. Mehta, S. Mazumdar, S. Singh, Magnetic adsorbents for the treatment of water/ wastewater—a review, J. Water Process Eng. 7 (2015) 244–265. [6] Y.M. Slokar, A.M. Le Marechal, Methods of decoloration of textile wastewaters, Dyes Pigm. 37 (1998) 335–356. [7] C. Fernández, M.S. Larrechi, M.P. Callao, An analytical overview of processes for removing organic dyes from wastewater effluents, TrAC, Trends Analyt. Chem. 29 (2010) 1202–1211. [8] Y. Deng, R. Zhao, Advanced oxidation processes (AOPs) in wastewater treatment, Curr. Pollut. Rep. 1 (2015) 167–176. [9] K. Ayoub, E.D. Van Hullebusch, M. Cassir, A. Bermond, Application of advanced oxidation processes for TNT removal: a review, J. Hazard. Mater. 178 (2010) 10–28. [10] N.H. Ince, D.T. Gönenç, Treatability of a textile azo dye by UV/H2O2, Environ. Technol. 18 (1997) 179–185. [11] S.-F. Kang, C.-H. Liao, S.-T. Po, Decolorization of textile wastewater by photoFenton oxidation technology, Chemosphere 41 (2000) 1287–1294.
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