Access the full text.
Sign up today, get DeepDyve free for 14 days.
Loading next page...
/lp/de-gruyter/preparation-characterization-of-a-ceria-loaded-carbon-nanotubes-1Ll8hep6oR
- Publisher
- de Gruyter
- Copyright
- Copyright © 2016 by the
- ISSN
- 2083-4810
- eISSN
- 2083-4810
- DOI
- 10.1515/aep-2016-0015
- Publisher site
- See Article on Publisher Site
Abstract
A ceria loaded carbon nanotubes (CeO2/CNTs) nanocomposites photocatalyst was prepared by chemical precipitation, and the preparation conditions were optimized using an orthogonal experiment method. HR-TEM, XRD, UV-Vis/DRS, TGA and XPS were used to characterize the photocatalyst. Nitrogen adsorption-desorption was employed to determine the BET specific surface area. The results indicated that the photocatalyst has no obvious impurities. CeO2 was dispersed on the carbon nanotubes with a good loading effect and high loading efficiency without agglomeration. The catalyst exhibits a strong ability to absorb light in the ultraviolet region and some ability to absorb light in the visible light region. The CeO2/CNTs nanocomposites photocatalyst was used to degrade azo dye Acid Orange 7 (40 mg/L). The optical decolorization rate was 66.58% after xenon lamp irradiation for 4 h, which is better than that of commercial CeO2 (43.13%). The results suggested that CeO2 loading on CNTs not only enhanced the optical decolorization rate but also accelerated the separation of CeO2/CNTs and water. Introduction Ceria (CeO2) is one of the rare earth oxides that has widely been used in the water gas shift reaction (Rodriguez et al. 2007, Vindigni et al. 2011), automotive three-way catalytic converters (Matsumoto 2004), green phosphors (Long et al. 2006), oxygen storage material (Campbell et al. 2005, Esch et al. 2005), solid fuel cells (Park et al. 2000) and antioxidants in biological model systems (Karakoti et al. 2010, Singh et al. 2011) due to its unique cubic fluorite structure (Trovarelli 2002), oxygen storage capacity (Walton 2011) and high activity. The photocatalytic oxidation performance of ceria is the focus of research due to its relatively numerous oxygen vacancies and low redox potential (Zhang et al. 2010). Zhai et al. (Zhai et al. 2007) employed homemade CeO2 nanocrystals as a catalyst for the degradation of 10 mg/L of acid black dye in water in sunlight, and the decolorization rate was 97%. Ji et al. (Ji et al. 2009) and Feng et al. (Feng et al. 2013) used CeO2 as a photocatalyst to degrade acid orange II, and its photocatalytic ability was better than that of commercial TiO2 (P25). Because ceria exhibits excellent photocatalytic performance, it is used as a support for loading other catalysts or as the main photocatalyst when it is loaded on other materials. The ability to load nanosized ceria particles on catalyst supports that have a high specific surface area and stability is required (Guerrero-Ruiz 1994, Soria et al. 1996, Trovarelli et al. 1995). Because carbon nanotubes have a large specific surface area and specific catalytic property as well as due to the advances in nanotube-walls functionalized research, much interest has been focused on CNTs for use in catalytic chemistry since their discovery in 1991 (Iijima 1991). CNTs exhibit high thermal and chemical stability due to their cylindrical layered, hollow tube nanostructures, which allows them to act as a catalysts to prepare loaded nanometals and metal oxide particles (Peng et al. 2005, Planeix et al. 1994). Researchers from the Chemistry Institute of Chinese Academy of Sciences used CNTs as a support for loading rare earth oxide to prepare CNTs-Eu2O3 one-dimensional composite catalysts in 2004 (Fu et al. 2004). Kang et al. (Kang et al. 2005) loaded platinum on CNTs to enhance the structural stability and electronic property of the CNTs. In addition, Rao et al. (Rao et al. 2012) prepared a CeO2/CNTs composite catalyst to dehydrogenate and oxidize ethylbenzene to styrene in a carbon dioxide atmosphere. Peng et al. (Peng et al. 2005) utilized CeO2 loaded with CNTs to remove arsenate in water. However, for the degradation of dye wastewater, researchers only used either CeO2 as a photocatalyst (Chen et al. 2011, Pouretedal et al .2010) or CNTs as an adsorbent to adsorb pollutants after pretreatment (Gao et al. 2013, Mishra et al. 2010). The use of CeO2/CNTs nanocomposites materials as photocatalysts for the degradation of azo dyes has not been reported. CeO2 nanoparticles and CNTs are both suspended in water, which makes their separation from water difficult. The decolorization efficiency of azo dye wastewater using CeO2 nanoparticles or CNTs is limited. In this paper, we loaded CeO2 on CNTs to prepare a nanocomposites catalyst that is easily separated from water and exhibits good decolorization performance to provide a new photocatalyst for the degradation of azo dyes. Materials and methods Various reagents employed [commercial CeO2, CeCl3 · 7 H2O, NaOH, polyethylene glycol (PEG 600), ethanol and a concentrated HNO3 aqueous solution] were purchased from Sinopharm Chemical Reagent Co., Ltd. in 99.99% purity or greater. Multi-walled carbon nanotubes with an average tube diameter of 80 nm and an average tube length of 20 m were purchased from Nanjing Xianfeng Co., Ltd. Deionized water was used to prepare the solvent and wash sample in this research. Preparation of nanocomposites photocatalyst Pretreatment of carbon nanotubes: 1 g of raw CNTs was weighed into a three-necked flask (250 mL capacity), and 100 mL of a concentrated HNO3 aqueous solution with a volume fraction of 65%~68% was added to the three-necked flask to treat the raw CNTs prior to purification. Then, the mixture was refluxed for 3 h at 125°C. Next, the mixture was successively washed with ethanol at first, then washed with deionized water until pH of 7 was obtained, and afterwards it was cooled to room temperature. Finally, the mixture was placed into a blast oven to dry for 24 h at 60°C and then ground prior to storage. Preparation of nanocomposites photocatalyst: A known quantity of pretreated carbon nanotubes was mixed with a known quantity of deionized water and then ultrasonically dispersed for 10 min. After adequately stirring for 1 h, a known quantity of CeCl3·7 H2O with a mass ratio of Ce to C of 2:3, 1:1, 4:3 or 3:2 and PEG 600 with a ratio of polyethylene glycol mass to the total mass of carbon nanotubes and CeCl3·7 H2O of 1% was added. A NaOH aqueous solution (6 g/L) was slowly added dropwise until the pH reached 11.0, 11.5, 12.0 or 12.5, followed by continuous stirring for 1 h, 2 h, 3 h or 4 h, respectively, at room temperature. Then, the mixture was filtered and washed until neutral and dry, which yielded a loose black powder. Finally, the black powder was dried and placed in a pipe furnace prior to being heated at a heating rate of 5°C/min to 400, 410, 420 or 430°C in air and maintained for 5 min, 10 min, 15 min or 20 min, respectively. Then, the CeO2/CNTs nanocomposites photocatalyst was obtained. Characterization of nanocomposites photocatalyst The morphological features and approximate size of the sample were observed using a Tecnai 12 transmission electron microscope (TEM) made by the Philips Company (Netherlands) and a FEI Tecnai G2 F30 S-Twin 300 KV high-resolution field emission transmission electron microscope (HR-TEM). The crystalline microstructure and phase composition of the sample were evaluated using German Bruker-AXS D8 Advance X-ray diffractometer (XRD), and test conditions were as follows: Cu target, diffuse radiation, tube voltage of 40 kV, tube current of 30 mA, scanning speed of 2 deg/min, scan range 2 of 20° ~80°, and slit width of 0.3 mm. The thermal weight loss of the sample during calcination was determined using an American Perkin Elmer Diamond TGA analyzer in air (TGA) at a heating rate of 5°C/min over a temperature range of 100~700°C. The absorbance threshold of the sample was recorded by a Japanese Shimadzu UV3600 UV-visible spectrophotometer (UV-Vis/DRS) with a scanning wavelength of 200~800 nm. The specific surface area of the sample was calculated using an American Micromeritics ASAP2020 M+C physical adsorption instrument. X-ray photoelectron spectroscopy (XPS) was performed to identify elements and analyze the valence of the elements in the samples using a Japanese ULVAC-PHT PHI 500 VersaProbe X-ray photoelectron spectrometer with a binding energy range of 0~1000 eV. Photocatalytic degradation experiment In a typical photocatalytic degradation experiments, 20 mL of Acid Orange 7 aqueous solution of 40 mg/L initial concentration was added to a test tube, then, the pH was adjusted to 5.0. And then, 10 mg of the CeO2/CNTs nanocomposites photocatalyst was introduced into the test tube and was dispersed by ultrasonic for 5 min. The test tube was then agitated for 1 h in the dark to reach adsorption-desorption equilibrium. Then, a 500 W xenon lamp was used as the simulated solar light source to irradiate the suspension for 4 h at a distance of 8 cm under uniform stirring. Finally, the absorbance of the supernatant was measured with a visible spectrophotometer at a wavelength of 484 nm to examine the optical decolorizing property of the CeO2/CNTs nanocomposites photocatalyst after being filtered by a microporous membrane (0.22 m). Settling performance experiment The settling performance of the CeO2/CNTs, commercial CeO2 and raw CNTs was tested using an ideal static settling experiment. In a typical settling performance experiment, the suspended particles were evenly dispersed in deionized water and then injected into a settling column. The particles were sampled at the same height, H, of the settling column (starting from the bottom of the settling column, H is 10 cm), at different times, t, followed by measuring the particulate matter concentration, C. Finally, the sedimentation curves were depicted as plots of the concentration ratio, C/C0 (C0 represents initial concentration of particulate matter), as a function of the corresponding settling velocity, u (=H/t). Therefore, the total removal of the suspended particles at a known settling velocity was calculated from the plots. Results and discussion Results and analysis of the orthogonal test The stirring reaction time, cerium carbon ratio, pH, calcination temperature and holding time were investigated in this study, and the optical decolorization rate of Acid Orange 7 as evaluation index based on single factor experiments (data not shown) and designed orthogonal test program of five factors and four levels were used to determine the optimum parameters for the preparation of the CeO2/CNTs nanocomposites photocatalyst. The results and analysis of the orthogonal test for the preparation of CeO2/CNTs nanocomposites photocatalyst are reported in Tables 1 and 2. As shown in Table 1, the test results indicated that the highest rate of optical decolorization was observed for No. 11 (62.14%). However, No. 11 may not be the optimal choice. Therefore, further analysis is required. Level difference analysis was used to investigate the level changes of the various factors that impact the tested items. This verification experiment was repeated twice for each of the two schemes mentioned above, and the results indicated that the optical decolorization rates of No. 11 were 62.15% and 61.26%. In addition, the optical decolorization rates for Group A were 66.58% and 68.13%. Obviously, the optical decolorization rates of Group A are still higher than those of No. 11. Therefore, the optimized conditions for the preparation of the CeO2/CNTs nanocomposites photocatalyst are as follows: stirring reaction time of 4 h, cerium carbon ratio of 4:3, pH of 11.5, calcination temperature of 410°C and holding time of 5 min. These conditions are used to prepare the photocatalysts employed in the rest of the experiments. Characterization of the nanocomposites photocatalyst TEM/HRTEM Fig. 1 shows the TEM and HR-TEM images of raw CNTs and the CeO2/CNTs nanocomposites photocatalyst. As shown in Fig. 1a (and inset), the tube-body of the CNTs is clean and well distributed after pretreatment. The TEM images of the CeO2/CNTs nanocomposites photocatalyst are shown in Fig. 1b (and inset), which indicates that the diameter of the CNTs were When the level difference (R) is larger, there is a greater impact on the factors that vary within a target range for testing indicator values, and the correct factor is the one with the primary influence. The results in Table 2 indicate that the level difference of the calcination temperature is the largest (21.777), which indicated that it has the largest influence on the preparation. Similarly, the level difference for the pH is the smallest (3.947), which indicated that the pH has the smallest effect on the preparation. Therefore, the order of effect of each factor on the photodecolorization rate from largest to smallest is as follows: calcination temperature > holding time > stirring reaction time> cerium carbon ratio > pH. We determined the theoretical optimal solution (Group A) based on the largest mean value of each factor (i.e., stirring reaction time of 4 h, cerium carbon ratio of 4:3, pH of 11.5, calcination temperature of 410°C and holding time of 5 min). The choice of the optimal scheme requires a comprehensive analysis and verification test. The preceding analysis indicated that the optimum parameters for the preparation of the CeO2/CNTs nanocomposites photocatalyst include either "No. 11" or Group A. However, the best choice will need to be determined through further testing. Table 1. Results from the orthogonal testt No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 A 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 B 2:3 1:1 4:3 3:2 2:3 1:1 4:3 3:2 2:3 1:1 4:3 3:2 2:3 1:1 4:3 3:2 C 11.0 11.5 12.0 12.5 11.5 11.0 12.5 12.0 12.0 12.5 11.0 11.5 12.5 12.0 11.5 11.0 D 400 410 420 430 420 430 400 410 430 420 410 400 410 400 430 420 E 5 10 15 20 20 15 10 5 10 5 20 15 15 20 5 10 Photodecolorization rate (%) 51.73 52.32 23.67 22.41 24.82 24.36 54.13 57.10 39.96 41.82 62.14 53.26 48.85 54.94 52.60 42.99 A: stirring reaction time (h); B: cerium carbon ratio; C: pH; D: calcination temperature (°); E: holding time (min). Table 2. Analysis of the results from the orthogonal test Levels k1 k2 k3 k4 R A 37.532 40.102 49.295 49.845 12.313 B 41.340 43.360 48.135 43.940 6.795 C 45.305 45.750 43.918 41.803 3.947 D 53.515 55.102 33.325 34.832 21.777 E 50.813 47.350 37.535 41.078 13.278 A: stirring reaction time (h); B: cerium carbon ratio; C: pH; D: calcination temperature (°); E: holding time (min). Fig. 1. TEM images of raw CNTs (a), CeO2/CNTs (b), HRTEM images of CeO2/CNTs (c&d) and SAED pattern of CeO2/CNTs (c, inset) significantly thicker after loading. In addition, the CeO2 particles are uniformly loaded without agglomeration, which indicates that the product has a high loading efficiency. As shown in the HRTEM images of the CeO2/CNTs nanocomposites photocatalyst in Fig. 1c, the CeO2 particles are loaded on the CNTs. In addition, the diameter of the CeO2 particle is 6 nm~10 nm with a single size distribution, which corresponds to the nanoparticle. The results in Fig. 1c (inset) indicate that the SAED pattern consists of a ring pattern. This typical polycrystalline ring pattern corresponds to the face-centered cubic polycrystalline structure of CeO2 (Zhang et al. 2007). Fig. 1d shows a single CeO2 nanoparticle, and the CeO2 lattice fringes are clearly visible. The observed lattice spacing (i.e., 0.31 nm and 0.27 nm) are in good agreement with the calculated ones for the (111) and (200) crystal planes of the cubic fluorite phase of CeO2 (Sathish et al. 2011). XRD Fig. 2 shows the XRD pattern of the CeO2/CNTs nanocomposites photocatalyst. As shown in Fig. 2, the main peaks corresponded to the graphene structure of CNTs (002) and the face-centered cubic structure of CeO2 [(111), (200),(220), (311), (222), (400), (331) and (420)], and no impurity peaks were observed. Therefore, no new substances were generated during the preparation. In addition, all of the main peaks of CeO2 were consistent with the standard cubic fluorite structure of CeO2 (PDF 34-0394), indicating that the prepared CeO2 particles have a polycrystalline face-centered cubic structure and exist as CeO2 (Chen et al. 2012), which is in agreement with the analysis of the SAED spectra. The CeO2 particle size calculated using the Scherrer equation (Chen et al. 2011) for the strongest peak (111) was 8.53 nm, which was in good agreement with the results of the particle diameter (6~10 nm) obtained from the analysis of the TEM images. ( 111 ) ( 220 ) ( 002 ) ( 200 ) ( 311 ) 50 2 ( o) Fig. 2. XRD pattern of the CeO2/CNTs TGA Fig. 3 shows the TG curves of the raw CNTs and the nanocomposites photocatalyst precursor CNTs-Ce (OH)3. The TG curve of the CNTs (grey line) indicated that the CNTs exhibited good heat resistance during calcination, and almost no loss of quality was observed when the temperature was lower than 500°C. When the temperature was higher than 550°C, the weight of the CNTs rapidly decreased, which indicated that the CNTs were being rapidly oxidized. When the temperature reached approximately 580°C, the CNTs completely disappeared. The TG curve of the nanocomposites photocatalyst precursor CNTs-Ce (OH) 3 (black line) indicated a weight loss with three steps. The first step occurs at 100~360°C due to the weight loss of precursor dehydration with a mass reduction rate of 3.55%. In the second step from 360°C to 520°C, the precursor weight decreased significantly as the temperature increased, which may be due to dehydrogenation ( 331 ) ( 420 ) ( 222 ) ( 400 ) the xenon lamp simulating solar light to test its photocatalytic property is feasible. The BET test results of the nanocomposites photocatalyst revealed that the specific surface area of CeO2/CNTs was 67.82 m2/g, which is nearly 70% higher than the specific surface area of raw CNTs (i.e., 40.00 m2/g) and higher than that of 23.53% reported in the literature (Peng et al.2005). Therefore, an increase in the specific surface area was achieved, resulting in an increased adsorption capacity of the nanocomposites photocatalyst. of the precursor Ce(OH)3 to form CeO2, and the CNTs were oxidized by the generated CeO2 with a weight loss rate of 43.45%. When the temperature reached and exceeded 520°C, the sample weight did not vary, which indicated that Ce(OH)3 had been oxidized to CeO2 and the CNTs had been completely decomposed to water and carbon dioxide. It should be noted that the heat resistance of the CNTs will be reduced greatly due to the existence of Ce(OH)3 and the generation of CeO2 in the calcination process, which could reduce the oxidation temperature of CNTs to 360°C. The results of previous single factor impact tests obtained by our group (data not shown) confirmed that CNTs in the precursor would be completely decomposed with a calcination temperature of 430°C and a holding time of 20 min. Therefore, the calcination temperature and holding time of the precursor should not be too high and too long (Zhang et al. 2007). However, the precursor Ce(OH)3 cannot be entirely prevented from converting to CeO2. In addition, the generated CeO2 cannot form good crystals when the temperature is too low. Therefore, the calcination temperature should be higher than 400°C (Zhai et al. 2007), which is contradictory for preparing the nanocomposites photocatalyst. Further investigation is required to determine a balance. At a calcination temperature of 410°C, the weight loss rate of the precursors was 8.39%. After subtracting the water loss rate of 3.55%, the CNT loss rate was less than 5%, which is acceptable during the preparation process. In addition, the calcination temperature of 410°C is reasonable and feasible based on the above analysis of TEM and XRD. Wavelength (nm) Fig. 4. UV-Vis spectrum of CeO2/CNTs 100 80 Weight loss (%) 60 40 20 0 100 200 300 400 500 600 700 Temperature ( ) CNTs-Ce(OH) CNTs Fig. 3. TG curve of the CeO2/CNTs precursor UV-Vis/ DRS and specific surface area measurements Fig. 4 shows the UV-Vis diffuse reflectance spectra of the CeO2/CNTs nanocomposites photocatalyst. As shown in Fig. 4, the prepared nanocomposites photocatalyst exhibited a strong ability to absorb light in the ultraviolet region. In addition, the band gap of the product was observed at approximately 450 nm after making the outside tangent line of the DRS curve. According to the formula Eg = 1240/, the corresponding band gap is 2.76 eV, which is slightly lower than that of 2.81eV, 2.95eV and 3.03eV reported in the literature (Chen et al. 2012, Feng et al. 2013, Zhao et al.2013) and theoretically results in better efficiency of light utilization. The nanocomposites photocatalyst also exhibited some capacity to absorb light in the visible light region, which implies that the utilization of XPS Fig. 5 shows the X-ray photoelectron spectra of the CeO2/CNTs nanocomposites photocatalyst. The wide spectrum (Fig. 5a) of CeO2/CNTs only shows the main peaks of the three elements (Ce, O and C) without any other elements, indicating that the sample was not contaminated by other elements during preparation and possesses high purity. Based on the XRD analysis, no incompletely reacted precursors were observed in the product. Fig. 5b shows the XPS Ce 3d spectrum of CeO2/CNTs. As shown in Fig. 5b, the peaks at 882, 898, 900.5, 907.3 and 916.5 eV can be attributed to Ce (IV), and the peaks at 882 and 898eV correspond to the Ce (IV) 3d94f1 and 3d94f0 final state, respectively. The spin-orbit splitting appears at 900.5, 907.3 and 916.5 eV, which corresponds to 3 u peaks of Ce (3d)3/2, indicating that the Ce species in the nanocomposites photocatalyst has a IV valence and exists as CeO2 (Mei et al. 2010, Park et al. 1996, Zhang et al. 2010). In addition, the O 1s spectrum of the prepared CeO2/CNTs is shown in Fig. 5c. As shown in Fig. 5c, the O 1s peak is primarily composed of two peaks centered at 529.1 and 531.4 eV, which are due to the lattice oxygen of CeO2(Rao et al. 2012) and the hydroxyl oxygen on the surface of CNTs or adsorbed oxygen from CeO2(Lakshminarayanan et al. 2004). In addition, the core level peak, which was located at 529.1 eV, indicated the existence of O2- (Zhang et al. 2007). In addition, the compound has formed a complete CeO2 lattice structure, which is consistent with the XRD and TEM analysis mentioned above. Therefore, the binding energies of Ce 3d and O 1s detected by XPS are consistent with those of standard CeO2. Photocatalytic Performance Testing The curves for the variation in the Acid Orange 7 concentration during dark adsorption and photocatalytic degradation in the presence of CeO2/CNTs, commercial CeO2 and raw CNTs are shown in Fig. 6a. As shown in Fig. 6a, the rates of Acid Orange 7 adsorption on the three materials in the dark are fast, and the adsorption-desorption equilibrium was achieved in the first 30 minutes. The adsorption efficiency of Acid Orange 7 on the CeO2 particles without a hole-like or tubular structure was significantly lower than that on CNTs and CeO2/CNTs. In addition, the total decolorization rate of Acid Orange 7 by the CeO2/CNTs nanocomposites photocatalyst was as high as 80.58%, and the optical decolorization rate was 66.58% after xenon lamp irradiation for 4 h, which is better than that of commercial CeO2 (i.e., 43.13%). This result may be due to the adsorption capacity of commercial CeO2 being lower than that of CeO2/CNTs, which resulted in the Acid Orange 7 molecules being more weakly adsorbed on the surface of commercial CeO2 and affecting its subsequent optical decolorization performance (Chen et al. 2012, Ayanda et al. 2015). Settling Performance Testing Fig. 6b shows the sedimentation curves of CeO2/CNTs, commercial CeO2 and raw CNTs in water. Based on the integral algorithm (Tang et al.2006), the total removal rate of the CeO2/ /CNTs suspended particles at a settling velocity of 1.5 cm/min is 59.83%, which is higher than that of CNTs (16.60%) and commercial CeO2 (44.40%), indicating that the CeO2/CNTs prepared in this research exhibits good settling performance accelerating its separation from water. Ce3d C1s Ce4d O1s 900 Binding Energy (eV) Binding Energy (eV) c 529.1 Binding Energy (eV) Fig. 5. XPS wide spectrum (a), Ce 3d spectrum (b) and O 1s spectrum (c) of CeO2/CNTs (a) Decolorization rate (%) 100 dark 80 light (b) 1,0 0,8 0,6 C / C0 40 CeO /CNTs 20 CeO CNTs 0 -60 0 60 120 180 240 0,4 CeO /CNTs CeO CNTs 0,2 0,0 0,0 0,5 1,0 1,5 2,0 2,5 3,0 Settling rate (cm / min) Fig. 6. Changes in the Acid Orange 7 concentration during dark adsorption and photocatalytic degradation (a) and sedimentation curves (b) in the presence of CeO2/CNTs, commercial CeO2 and raw CNTs Guerrero-Ruiz, A. (1994).Carbon monoxide hydrogenation over carbon supported cobalt or ruthenium catalysts. Promoting effects of magnesium, vanadium and cerium oxides, Appllied Catalysis, A: General, 120, pp. 7183. Iijima, S. (1991). Helical microtubules of graphitic carbon, Nature, 354, pp. 5658. Ji, P.F., Zhang, J.L., Chen, F. & Anpo, M. ( 2009). Study of adsorption and degradation of acid orange 7 on the surface of CeO2 under visible light irradiation, Applied Catalysis B: Environmental, 85, pp. 148154. Kang, M., Bae, Y.S. & Lee, C.H. ( 2005). Effect of heat treatment of activated carbon supports on the loading and activity of Pt catalyst, Carbon, 43, pp. 15121516. Karakoti, A., Singh, S. & Dowding, J.M. (2010). Redox-active radical scavenging nanomaterials, Chemical Society Reviews, 39, pp. 44224432. Lakshminarayanan, P.V., Toghiani, H. & Pittman, Jr, C.U. (2004). Nitric acid oxidation of vapor grown carbon nanofibers, Carbon, 42, pp. 24332442. Long, Z. Q., Ren, L., Zhu, Z.W., Cui, D.L., Zhao, N., Li, M.L., Cui, M.S. & Huang, X.W. (2006). Synthesis of LaPO4: Ce, Terbium by Co-Precipitation Method, Journal of Rare Earths, 24, pp. 137140. Matsumoto, S. (2004). Recent advances in automobile exhaust catalysts, Catalysis Today, 90, pp. 183190. Mei, Y., Yan, J.P. & Nie, Z.R. (2010). XPS study on the influence of calcination conditions to cerium ion valence, Spestroscopy and Spectral Analysis, 30, pp. 270273. (in Chinese) Mishra, A.K., Arockiadoss, T. & Ramaprabhu, S. (2010). Study of removal of azo dye by functionalized multi walled carbon nanotubes, Chemical Engineering Journal, 162, pp. 10261034. Park, P.W. & Ledford, J.S. (1996). Effect of crystallinity on the photoreduction of cerium oxide: A study of CeO2 and Ce/Al2O3 catalysts, Langmuir, 12, pp. 17941799. Park, S., Vohs, J.M. & Gorte, R.J. (2000). Direct oxidation of hydrocarbons in a solid-oxide fuel cell, Nature, 404, pp. 265267. Peng, X.J., Luan, Z.K., Ding, J., Di, Z.C., Li, Y.H. & Tian, B.H. (2005). Ceria nanoparticles supported on carbon nanotubes for the removal of arsenate from water, Materials Letters, 59, pp. 399403. Planeix, J.M., Coustel, N., Coq, B., Brotons, V., Kumbhar, P.S., Dutartre, R., Geneste, P., Bernier, P. & Ajayan, P.M. (1994). Application of carbon nanotubes as supports in heterogeneous catalysis, Journal of the American Chemical Society, 116, pp. 79357936. Pouretedal, H.R. & Kadkhodaie, A. (2010). Synthetic CeO2 nanoparticle catalysis of methylene blue photodegradation: kinetics and mechanism, Chinese Journal of Catalysis, 31, pp. 13281334. Rao, R., Zhang, Q.Y., Liu, H.D., Yang, H.X., Ling, Q., Yang, M., Zhang, A.M. & Chen, W. (2012). Enhanced catalytic performance of CeO2 confined inside carbon nanotubes for dehydrogenation of ethylbenzene in the presence of CO2, Journal of Molecular Catalysis A: Chemical, 363364, pp. 283290. Rodriguez, J.A., Ma, S., Liu, P., Hrbek, J., Evans, J. & Pérez, M. (2007). Activity of CeOx and TiOx nanoparticles grown on Au (111) in the water-gas shift reaction, Science, 318, pp. 17571760. Sathish, M., Miyazawa, K. & Ye, J. (2011). Fullerene nanowhiskers at liquid-liquid interface: A facile template for metal oxide (TiO2, CeO2) nanofibers and their photocatalytic activity, Materials Chemistry and Physics, 130, pp. 211217. Singh, S., Dosani, T., Karakoti, A.S., (2011). A phosphate-dependent shift in redox state of cerium oxide nanoparticles and its effects on catalytic properties, Biomaterials, 32, pp. 67456753. Soria, J., Coronado, J.M. & Conesa, J.C. (1996). Spectroscopic study of oxygen adsorption on CeO2/-Al2O3 catalyst supports, Journal of the Chemical Society, Faraday Transactions, 92, pp. 16191626. Conclusions CeO2 can be loaded on CNTs uniformly to prepare a CeO2/ /CNTs nanocomposites photocatalyst. The optimized conditions are as follows: stirring reaction time, 4 h; cerium:carbon ratio, 4:3; pH, 11.5; calcination temperature, 410°C; and holding time, 5 min. The prepared nanocomposites photocatalyst has no significant impurities, and the size of the CeO2 loaded on the CNTs is 6~10 nm. In addition, CeO2 was dispersed on the carbon nanotubes with a good loading effect and a high loading efficiency. The specific surface area of CeO2/CNTs is 67.82 m2/g, which is nearly 70% higher than that of raw CNTs (i.e., 40.00 m2/g), and CeO2/CNTs exhibits a good adsorption capacity. In addition, CeO2/CNTs has a strong ability to absorb light in the ultraviolet region and exhibits some capacity to absorb light in the visible light region. The optical decolorization ratio of CeO2/CNTs to degrade azo dye Acid Orange 7 (40 mg/L) was as high as 66.58% after xenon lamp irradiation for 4 h, which is better than that of commercial CeO2 (43.13%) with an improved settling performance. This result indicates that CeO2 loaded on CNTs can enhance the optical decolorization rate and accelerate the separation of CeO2/CNTs and water. Therefore, the prepared CeO2/CNTs nanocomposites photocatalytic material exhibits potential for application in the photocatalytic treatment of azo dyes in water. Acknowledgments This work was financially supported by Jiangsu General College Graduate Students Research and Innovation Program, China (CXZZ13_0723).
Journal
Archives of Environmental Protection – de Gruyter
Published: Jun 1, 2016