Article pubs.acs.org/IECR
An Outward Coating Route to CuO/MnO2 Nanorod Array Films and Their Efficient Catalytic Oxidation of Acid Fuchsin Dye Zeheng Yang,* Yumei Yang, Xiao Zhu, Gongde Chen, and Weixin Zhang* School of Chemical Engineering, Hefei University of Technology and Anhui Key Laboratory of Controllable Chemical Reaction & Material Chemical Engineering, Hefei, Anhui 230009, People’s Republic of China S Supporting Information *
ABSTRACT: An outward coating method has been successfully employed to prepare CuO/MnO2 nanorod array films based on the impregnation of Cu(OH)2 nanorod array films with manganese nitrate aqueous solution and heat post-treatment. The asprepared CuO/MnO2 nanorod array films as heterogeneous catalysts successfully address such issues as easy agglomeration, difficult separation, and possible secondary pollution related to powder catalysts. Furthermore, they exhibit catalytic oxidation activity for the degradation of acid fuchsin (AF) dye in aqueous solution superior to that of bare CuO nanorod array films in the presence of H2O2, because of the synergistic effects of both CuO and MnO2. The effects of the initial concentration of aqueous AF solution and H2O2 dosage on the catalytic oxidation performance were evaluated, indicating that the degradation ratio of AF can reach up to 94.05%. Life-cycle performance and scaleup of the catalytic oxidative degradation process demonstrate the durability and potential engineering application of CuO/MnO2 nanorod array films in dye wastewater treatment.
1. INTRODUCTION Industrial dyestuffs containing various organic compounds lead to increasing environmental concerns, and their degradation has, therefore, attracted much attention.1 Dyes usually have complex aromatic molecular structures that make them more stable and more difficult to degrade. As an acid stain, acid fuchsin (AF) is mainly used for biological staining and connective tissue staining. It belongs to the class of triphenylmethanes and highly conjugated polymer systems of dyes, which are difficult to degrade in wastewater through traditional chemical and biological approaches.2 Advanced oxidation processes (AOPs) are based on the generation of hydroxy radicals (•OH) that oxidize refractory organic pollutants into low-toxicity or nontoxic small molecules, even into CO2 and H2O quickly without any choice. AOPs usually include Fenton oxidation (homogeneous and heterogeneous), ozone oxidation, photoelectrocatalytic oxidation, supercritical water oxidation, and wet air oxidation, among others. However, homogeneous Fenton oxidation is restricted by low utilization of H2O2, introduction of iron ions, difficulties of reutilization, and additional costs. Ozonation is a powerful technology, but the limited solubility and short lifetime of ozone lead to high energy consumption.3 Nevertheless, if hydroxyl radicals are generated photochemically, the cost of the process is quite high. Supercritical water oxidation and wet air oxidation are restricted by high-pressure and -temperature conditions in the catalytic oxidative degradation process. To overcome these disadvantages, increasing attention has been paid to research on heterogeneous Fenton oxidation, which is highly efficient, provides energy savings, and generates no secondary pollution. Transition-metal oxides have been widely studied as heterogeneous Fenton oxidation catalysts, such as MnO2,4 Mn3O4,5 Fe2O3,6 NiO,2 Co3O4,7 and CuO.8 Both copper oxides8 and manganese oxides9 have good catalytic oxidation © 2014 American Chemical Society
activities for the decomposition of H2O2, and the produced hydroxyl radicals (•OH) have a high oxidizing ability for decomposing toxic and refractory organic pollutants from water. Recently, a CuO/MnO2 composite has been reported to take advantage of the synergistic effects of the two oxides for the promotion of H2O2 decomposition. Angeles-Hernández et al.10 studied the oxidation of quinoline over MnO2/CuO catalyst under supercritical conditions and demonstrated that the removal of quinoline reached a value close to 98%. Qian et al.11 synthesized a series of CuO/MnO2 catalysts with different CuO loadings by the incipient-wetness impregnation method and demonstrated that the CuO/MnO2 interface could serve as the active sites for CO oxidation on CuO/MnO2 catalysts. Li et al.12 successfully synthesized MnO2 nanoparticles loaded onto the surface of CuO nanosheets through an immersion approach followed by heat post-treatment and found that MnO2-loaded CuO nanosheets could be expected to be a promising catalyst for heterogeneous catalytic ozonation because of its composite phases, high dispersity, and large specific surface area. Nevertheless, these composite catalysts exist in powder form, and there are no reports on CuO/MnO2 composite catalysts in the form of nanoarray films. It is known that the application of powder nanocatalysts usually faces such issues as easy agglomeration, difficult separation, and possible secondary pollution. Particularly, for the Fenton catalytic oxidation system of transition-metal oxide with H2O2, the decomposition of H2O2 will produce a lot of O2 bubbles that tend to blow the nanocatalyst powder to the surface of the solution through foaming, thus reducing the contact area of the catalyst with the system and resulting in Received: Revised: Accepted: Published: 9608
January 28, 2014 May 8, 2014 May 15, 2014 May 15, 2014 dx.doi.org/10.1021/ie500358p | Ind. Eng. Chem. Res. 2014, 53, 9608−9615
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mL of CH3CH2OH, and 36 mL of distilled water were added into a stainless steel autoclave of 50 mL capacity, and then the autoclave was sealed and kept at 160 °C for 24 h. The product was collected by washing, filtering, and drying and was subsequently calcined at 350 °C for 4 h in the air to obtain pulverous MnO2 nanorod catalyst. 2.5. Characterization of the Samples. The samples were characterized by powder X-ray diffraction on a Rigaku D/maxγB X-ray diffractometer with a Cu Kα radiation source (λ = 0.154178 nm) operated at 40 kV and 80 mA. Transmission electron microscopy (TEM) images were recorded on a Hitachi H-800 transmission electron microscope at an accelerating voltage of 200 kV. Field-emission scanning electron microscopy (FESEM) and scanning electron microscopy (SEM) measurements were taken on FEI Sirion-200 and JSM-6490LV scanning electron microscopes, respectively. Elemental mappings and field-emission transmission electron microscopy (FETEM) images were acquired on a JEM-2100F field-emission transmission electron microscope. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB 250 spectrometer using an Al Kα X-ray (hν = 1486.6 eV) source operated at 150 W with an analyzer pass energy of 20 eV, and thermogravimetric analysis (TGA) was carried out on a Netzsch TG209F3 thermogravimeter analyzer. Fourier transform infrared (FTIR) spectroscopy analysis was performed on a Shimadzu IR440 spectrometer in the mid-IR range of 400− 4000 cm−1. The sample for FTIR analysis of AF dye was prepared by tableting a mixture of pure KBr and AF dye powder and then fixed in the sample holder for analysis. Analogously, the concentrated aqueous AF solution after catalytic oxidative degradation was wetted onto tableted pure KBr for analysis. 2.6. Catalytic Oxidation Experiments. The catalytic activities of the as-prepared samples were evaluated for the degradation of acid fuchsin (AF) in aqueous solution in the presence of H2O2. For the convenience of testing, the catalytic oxidation reaction was carried out in a quartz cuvette containing 3 mL of aqueous AF solution. CuO/MnO2 or CuO nanorod array film was vertically immersed in the solution. To maintain the same initial concentration of aqueous AF solution, the volume of the mixed solution was fixed at 3.5 mL in the testing process through the addition of a certain amount of H2O2 and/or water. After the addition of H2O2 solution (30 wt %), the concentration of residual AF in the solution was examined at a given time interval by detecting the characteristic peak at 540 nm on a ultraviolet−visible spectrophotometer (UV-754, Precision Scientific Instrument Co. Ltd., Shanghai, China). The degradation ratio of aqueous AF solution was calculated according to the Lambert−Beer law. In addition, a linear calibration curve for the dye concentrations was obtained by measuring the peak intensity of a series of standard aqueous AF solutions at 540 nm, and the subsequent degradation ratio was measured discontinuously by calculating the relative variation ratio of absorbance, which is basically directly proportional to the concentration of the aqueous AF dye solution.
poor catalytic oxidation performance. Nanostructured catalysts immobilized on certain substrates can overcome the problems mentioned above to some extent. We have reported wellaligned TiO2 nanotube arrays on a copper substrate can be used as photocatalysts to degrade rhodamine B with a removal ratio of 99.3%.13 Similarly, well-aligned ZnSe nanotube arrays on a zinc substrate can be used as photocatalysts to degrade methyl orange with a removal ratio of 76.5%.14 Herein, we demonstrate a simple route to the synthesis of CuO/MnO2 nanorod array films. A MnO2 layer has been coated on CuO nanorod arrays by the immersion of Cu(OH)2 nanorod arrays in Mn(NO3)2 aqueous solution, followed by calcination in nitrogen atmosphere. The as-prepared CuO/ MnO2 nanorod array films as heterogeneous catalysts not only address the issues peculiar to powder catalysts successfully, but also exhibit catalytic activity for the degradation of AF dye in aqueous solution superior to that of bare CuO nanorod array films in the presence of H2O2 because of the synergistic effects of CuO and MnO2. The effects of the initial concentration of AF solution and the H2O2 dosage on the catalytic oxidation performance were evaluated. Furthermore, the life-cycle performance of the CuO/MnO2 nanorod array films was investigated, and the stable catalytic activity found in scaled-up catalytic oxidation reaction system demonstrates the potential engineering application of CuO/MnO2 nanorod array films for wastewater treatment.
2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. and used as-received without any further purification. Stock solutions of sodium hydroxide (10 mol/L) and ammonia persulfate (1 mol/L) were prepared by dissolving NaOH and (NH4)2S2O8, respectively, in distilled water. An aqueous solution of manganese nitrate (50 wt %) was used as the manganese source. Ammonia solution (25 wt %) and high-purity copper foils (99.99%) were also used. 2.2. Preparation of Cu(OH)2 Nanorod Array Films. The synthesis of Cu(OH)2 nanorod array films grown on copper foil was described in a previous report.15 Typically, 10 mL of distilled water, 2 mL of NaOH solution (10 mol/L), 1.0 mL of (NH4)2S2O8 solution (1 mol/L), and 1.0 mL of ammonia solution (25 wt %) were added in turn to a bottle with a volume of 60 mL. A piece of copper foil (10 × 10 × 0.25 mm3) that had been ultrasonically cleaned in acetone, absolute ethanol, and distilled water in turn was immersed in the aboveprepared solution in the bottle at room temperature. After 40 min, the copper foil covered with a blue films was taken out; rinsed with distilled water and absolute ethanol, respectively; and dried in air. 2.3. Preparation of CuO and CuO/MnO2 Nanorod Array Films. The as-prepared Cu(OH)2 nanorod array films grown on copper foil were calcined in a nitrogen atmosphere at 250 °C for 3 h to obtain CuO nanorod array films. The procedure to synthesize CuO/MnO2 nanorod array films was performed as follows: The Cu(OH)2 nanorod array films were immersed in 10 mL of an aqueous solution of manganese nitrate (1 v/v%) for 8 h. Then, the sample was taken out, dried in air without rinsing, and calcined in nitrogen atmosphere at 250 °C for 3 h. 2.4. Preparation of Pulverous MnO2 Nanorod Catalyst. The synthesis of powder MnO2 catalyst was described in a previous report.16 In a typical procedure, 0.4 g of KMnO4, 0.4
3. RESULTS AND DISCUSSION 3.1. Structures and Morphologies of the Nanorod Array Films. Figure 1 shows XRD patterns of the samples on a copper substrate. The series of diffraction peaks in Figure 1a can be indexed to orthorhombic Cu(OH)2 (JCPDS 13-0420), except for those marked with circles, which come from the 9609
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Figure 1. XRD patterns of the samples: (a) Cu(OH)2 nanorod arrays, (b) CuO nanorod arrays, and (c) CuO/MnO2 nanorod arrays on copper substrate.
copper substrate. After calcination, the peaks presented in Figure 1b can be indexed to monoclinic CuO (JCPDS 481548), except for the peaks from the copper substrate. Figure 1c shows the XRD pattern of the sample obtained after impregnation of Mn(NO3)2 and subsequent calcination, which is similar to Figure 1b, and no obvious characteristic peaks can be observed for manganese oxides, probably because of their small amount. In addition, UV−vis spectrophotometry analysis was performed on a UV-2550 ultraviolet−visible spectrophotometer (CRXT Environment Technology Co. Ltd., Beijing, China) to characterize the optical absorption properties of the three samples, and the results are presented in Figure S1 (Supporting Information). Figure S1a (Supporting Information) presents a spectrum of Cu(OH)2 nanorod arrays, and the absorption spectral ranges are 230−400 and 550−800 nm. The CuO (Figure S1b, Supporting Information) and CuO/MnO2 (Figure S1c, Supporting Information) nanorod arrays have similar spectra and exhibit weak broad absorption peaks in the 400− 600 nm range, which is consistent with literature reports.17,18 From the analysis of the UV−vis diffuse reflectance spectra of the samples, no obvious difference can be seen between bare CuO and CuO/MnO2 composite in the range of 200−800 nm. Therefore, it is necessary to demonstrate the presence of MnO2 further by XPS, elemental mappings, and FETEM analysis. FESEM and TEM studies (Figure 2) were undertaken to characterize the morphologies and structures of the samples. Panels a and b of Figure 2 display FESEM and TEM images, respectively, of Cu(OH)2 nanorod array films. As can be seen, the Cu(OH)2 nanorods are well-aligned and have smooth surfaces with diameters of 400−500 nm and lengths of 5−10 μm. The FESEM (Figure 2c) and TEM (Figure 2d) images of the CuO nanorod arrays exhibit a morphology similar to that of the Cu(OH)2 nanorod arrays, but with many tiny cracks resulting from calcination. After impregnation and calcination of the Cu(OH)2 precursor, CuO/MnO2 nanorod arrays are obtained. Figure 2e shows an FESEM image of the CuO/MnO2 nanorod arrays, and Figure 2f shows a TEM image of a single nanorod. Similarly, the CuO/MnO2 sample still retains the nanorod-array morphology at a large scale, and a layer of MnO2 is uniformly produced on the surface of the nanorod (Figure 2f). The XRD pattern and a TEM image of the pulverous MnO2 nanorod catalyst are presented in Figure S2 (Supporting Information). All peaks in Figure S2a (Supporting Information) can be indexed to MnO2 (JCPDS 24-0375), and the pulverous
Figure 2. (a,c,e) FESEM and (b,d,f) TEM images of the samples: (a,b) Cu(OH)2 nanorod arrays, (c,d) CuO nanorod arrays, and (e,f) CuO/ MnO2 nanorod arrays on copper substrate.
MnO2 nanorods have diameters of 50−200 nm and lengths of over 10 μm (Figure S2b, Supporting Information). XPS measurements were made to characterize the CuO/ MnO2 nanorod arrays. The Mn 2p region in Figure 3a shows that the binding energies of the Mn 2p1/2 and Mn 2p3/2 peaks are located at 653.98 and 641.38 eV, respectively, attributed to Mn4+ (641.1−642.4 eV) in MnO2.19−21 The O 1s region in Figure 3b indicates that the binding energy of 529.68 eV can be ascribed to the lattice oxygen (O2−), and the higher-bindingenergy peak around 531.18 eV can be ascribed to surface oxygen adsorbed from the atmosphere.22 Elemental mappings and FETEM analysis (Figure 4) were performed to further confirm the existence of MnO2 in the CuO/MnO2 nanorods. Panels a−d of Figure 4 present an FETEM image of the CuO/MnO 2 nanorod and the corresponding elemental mappings for Cu, Mn, and O, respectively. It can be seen that all elements (Cu, Mn, and O) are uniformly distributed in the nanorods, and the content of Mn is approximately 1.91 at. % (Figure S3, Supporting Information), verifying that a small amount of MnO2 formed on the CuO nanorods. Panels e and f of Figure 4 show FETEM and HRTEM images, respectively, of a CuO/MnO2 nanorod. In Figure 4f (corresponding to the area marked by the circle in Figure 4e), the marked lattice fringe spacings of 0.236 and 0.246 nm correspond to the (111) lattice plane of monoclinic CuO (JCPDS 48-1548) and the (210) lattice plane of 9610
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Figure 3. XPS spectra of CuO/MnO2 nanorod arrays: (a) Mn 2p and (b) O 1s.
orthorhombic MnO2 (JCPDS 42-1316), respectively, confirming the coexistence of CuO and MnO2. These analyses confirm that a MnO2 layer was successfully loaded on the CuO nanorods. To determine the proper temperature for the formation of CuO/MnO2 nanorods and further verify the existence of MnO2, the Cu(OH)2 nanorod array with adsorbed manganese nitrate was scraped from the Cu substrate, and the thermal behavior was investigated by TGA. As shown in Figure 5, most of the weight loss occurs below 183 °C, which is mainly caused by the dehydration of Cu(OH)2. A small weight loss appearing from 183 to 222 °C is attributed to the decomposition of Mn(NO3)2. At temperatures above 250 °C, the weight loss is insignificant compared with the lower range, so 250 °C was chosen as the proper calcination temperature. The result is consistent with the report by Nossen, who prepared MnO2 by thermal decomposition of Mn(NO3)2 in a closed electric stove at 180 °C for 48 h.23 3.2. Catalytic Oxidation Activities of the Nanostructured Array Films. Acid fuchsin dye belongs to the triphenylmethane class and highly conjugated polymer systems with amino and sulfonic acid groups (its molecular structure is shown in Figure 6), which is difficult to degrade in wastewater. Heterogeneous Fenton oxidation is highly efficient, provides energy savings, and produces no secondary pollution. The hydroxy radicals (•OH) generated are recognized as being a highly reactive and powerful oxidant that can oxidize and decompose numerous hazardous compounds to CO2 and inorganic ions because of its high standard reduction potentials of 2.8 V in acidic solution and 1.8 V in neutral solution.24 During the catalytic oxidation reaction, AF dye and H2O2 are first adsorbed on the catalyst surface; then, H2O2 is catalyzed to generate hydroxyl radicals (•OH) to promote the degradation
Figure 4. (a) FETEM image of a single CuO/MnO2 nanorod. (b−d) Elemental mappings for (b) Cu, (c) Mn, and (d) O. (e) FETEM and (f) HRTEM images of the CuO/MnO2 nanorod.
Figure 5. TG and DTG curves of the Cu(OH)2 nanorod array films adsorbed with manganese nitrate for the formation of CuO/MnO2 nanorod arrays.
of organic compounds. In addition, decomposition of H2O2 also generates O2 bubbles, which can stir the reaction solution and are beneficial in enhancing diffusion of the reactants. The catalytic oxidation performance of CuO/MnO2 nanorod arrays in the oxidation of AF dye was studied in the presence of H2O2. Figure S4 (Supporting Information) shows that the presence of H2O2 has no effect on the characteristic absorption of AF dye at 540 nm. The linear calibration curve presented in Figure S5 (Supporting Information) has a correlation coefficient of 0.9995, indicating a good linear relation between 9611
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measured and are presented in Figure S6 (Supporting Information), and the results are consistent with Figure 7. Thereinto, the AF bands at 242 and 307 nm are masked by the strong absorption of H2O2 in the ultraviolet region. Possible intermediates related to the degradation process are not revealed here and are investigated in later FTIR analysis. The effect of dye concentration on the degradation ratio was studied by keeping the mass of catalyst and the dosage of H2O2 constant while varying the concentration of dye. As reported in Figure 8, the degradation ratios for aqueous AF solution at 5, Figure 6. Molecular structure of acid fuchsin dye.
the absorption peak intensity and the concentration of AF solution. To verify the catalytic oxidation performance of CuO/MnO2 nanorod array films, a series of comparison experiments were performed. Figure 7 shows that the CuO/MnO2 array film
Figure 8. Time-dependent degradation ratios of aqueous AF solution at different initial concentrations in the presence of a piece of CuO/ MnO2 nanorod array film and 0.05 mL of H2O2.
10, 15, 20, 25, and 30 mg/L were found to be 83.67%, 85.50%, 90.57%, 93.08%, 88.28%, and 79.59%, respectively. The degradation ratio first increased and then decreased with increasing AF dye concentration, which can be explained as follows: With increasing initial dye concentration, more dye molecules are adsorbed on the surface of the catalyst, and the highest catalytic activity can be achieved when the adsorption sites are saturated. However, as the dye concentration is increased further, the excess dye molecules have an inhibitive effect on the reaction of the dye molecules and •OH radicals because of a lack of effective contact between the dye molecules and H2O2 on surface of the catalyst, thus resulting in a decrease of the degradation ratio.25 According to the results presented in Figure 8, the removal ratio of AF dye at 20 mg/L is superior to those at the other concentrations. Figure 9 shows the effect of H2O2 dosage on the catalytic activity of the CuO/MnO2 nanorod arrays. The degradation ratio exhibits no obvious change but a slightly decreasing tendency in 2 h with increasing H2O2 dosage, which can be ascribed to self-quenching of •OH radicals upon addition of superfluous H2O2 to produce •HO2 radicals.26 Additionally, the peroxy radicals can react through other pathways to produce oxygen. That is, dilute solutions of H2O2 are economical for the highly efficient removal of organics from wastewater with nanorod array film catalysts. To examine the durability and reusability of the CuO/MnO2 nanorod arrays for water treatment, a life-cycle evaluation was performed, and the corresponding results are presented in Figure 10. All of the experiments were conducted under the same conditions (initial AF concentration, 20 mg/L; H2O2 dosage, 0.05 mL; catalytic oxidation reaction time, 8 h). After each run, the CuO/MnO2 nanorod array film was cleaned sequentially with deionized water and absolute ethanol and then dried in air. As observed, after 10 runs, there was no
Figure 7. Time-dependent degradation ratios of aqueous AF solution in the presence of (a) 3 mL of 20 mg/L AF, a piece of CuO/MnO2 nanorod array film, 0.05 mL of H2O2, and 0.45 mL of H2O; (b) 100 mL of 20 mg/L AF, 24 mg of pulverous MnO2 nanorods, and 5 mL of H2O2; (c) 3 mL of 20 mg/L AF, a piece of CuO nanorod array film, 0.05 mL of H2O2, and 0.45 mL of H2O; (d) 3 mL of 20 mg/L AF, a piece of CuO/MnO2 nanorod array film, and 0.5 mL of H2O; and (e) 3 mL of 20 mg/L AF, 0.05 mL of H2O2, 0.45 mL of H2O, and no catalyst.
catalyst achieved a high removal ratio of 94.05% after 8 h of degradation (Figure 7a) in the presence of 3 mL of 20 mg/L AF dye solution, a piece of CuO/MnO2 film (about 0.2 mg of CuO/MnO2), 0.05 mL of H2O2, and 0.45 mL of H2O, whereas the the pulverous MnO2 nanorod catalyst and the CuO array film exhibited inferior degradation ratios of 79.6% and 72.5% (Figure 7b,c), respectively. For the pulverous MnO2 nanorod catalyst, a higher dosage of 24 mg of MnO2 powder catalyst was used instead of the proportionately increased theoretical dosage of about 6 mg of CuO/MnO2 catalyst for 100 mL of 20 mg/L AF dye solution. Obviously, the CuO/MnO2 film catalyst shows a synergistic effect in the degradation of AF dye solution. For comparison, two blank tests with only CuO/MnO2 (Figure 7d) and only H2O2 (Figure 7e) were also conducted, and the corresponding degradation ratios were found to be 41.01% and 14.87%, respectively, which can probably be ascribed to the adsorption effect of the catalyst and the self-decomposition of H2O2 to produce hydroxyl radicals (•OH), respectively. The entire spectra of aqueous AF dye solutions in the degradation process of the above-mentioned comparison experiments (except that in Figure 7d with only CuO/MnO2) were 9612
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Figure 11. Time-dependent degradation ratios of aqueous AF solution (20 mL, 20 mg/L) in the scaled-up system: (a) six pieces of CuO/ MnO2 nanorod array film and 1 mL of H2O2, (b) six pieces of CuO nanorod array film and 1 mL of H2O2, (c) six pieces of CuO/MnO2 nanorod array film and 1 mL of H2O, and (d) 1 mL of H2O2 and no catalyst.
Figure 9. Time-dependent degradation ratios of aqueous AF solution (3 mL, 20 mg/L) in the presence of a piece of CuO/MnO2 nanorod array film and different dosages of H2O2.
H2O2 decomposition activity over CuO and MnO2 catalysts and confirmed that a favorable Mn3+/Mn4+ or Cu+/Cu2+ couple is necessary for the decomposition of H2O2, suggesting that, if the catalyst accepts an electron, the possible active site is Mn4+ or Cu2+ to yield a •HO2 radical (reactions 1 and 3) and, if the electron is donated, the active site is Mn3+ or Cu+ to yield a • OH radical (reactions 2 and 4). Hence, the combination of CuO and MnO2 is synergetic in the promotion of H2O2 decomposition to •OH radicals.
Figure 10. Cycling runs in the catalytic oxidative degradation of aqueous AF solution (3 mL, 20 mg/L) in the presence of a piece of CuO/MnO2 nanorod array film and 0.05 mL of H2O2.
distinct decrease in the AF degradation ratio, which varied from 94.05% in the first run to 90.28% in the 10th run. These results indicate that CuO/MnO2 nanorod array films exhibit good stability as catalysts. To determine the potential applicability of CuO/MnO2 nanorod array films for wastewater treatment, we further investigated a scaled-up system of the catalytic oxidation reaction, a schematic illustration of which is presented in Figure S7 (Supporting Information). Figure 11 shows the catalytic oxidation performance of the CuO/MnO2 nanorod array films in the scaled-up systems. Because the volume of the initial aqueous AF solution (20 mL, 20 mg/L) and the dosage of H2O2 (1 mL, 30 wt %) were both increased, six pieces of CuO/ MnO2 nanorod array film were used (Figure 11a) in the catalytic oxidation reaction. Correspondingly, three comparison experiments were also conducted in the same manner as described above. The results presented in Figure 11 indicate that the degradation ratio of AF dye could reach 94.32% in the scaled-up system, and there was no notable difference in the degradation ratio compared with that presented in Figure 7. The scaled-up experiments demonstrate the potential engineering application of the CuO/MnO2 nanorod array films for dye wastewater treatment. 3.3. Mechanisms for the Degradation of AF Dye Solution. Researchers8,9 have demonstrated the promotion of
Mn 4 + + H 2O2 + e− → Mn 3 + + •HO2 + H+
(1)
Mn 3 + + H 2O2 − e− → Mn 4 + + •OH + OH−
(2)
Cu 2 + + H 2O2 + e− → Cu+ + •HO2 + H+
(3)
Cu+ + H 2O2 − e− → Cu 2 + + •OH + OH−
(4)
MnOH(b) + H 2O + RSO3− ⇄ MnOH 2+SO3−R + OH−
(5)
CuOH(b) + H 2O + RSO3− ⇄ CuOH 2+SO3−R + OH−
(6)
MnOH(a) + Na + ⇄ MnO−Na + + H+
(7)
CuOH(a) + Na ⇄ CuO Na + H
(8)
+
−
+
+
27
Tamura et al. reported that oxide surfaces are covered with acid and base hydroxyl groups formed by the dissociative chemisorption of water molecules. A base hydroxyl group is protonated by releasing an OH− ion to become a positive site, MnOH2+ or CuOH2+ (reactions 5 and 6), to which a R SO3− ion is electrostatically adsorbed (RSO3− comes from the dissociation of an AF molecule RSO3Na). An acid hydroxyl group dissociates into a H+ ion and a negative site, MnO− or CuO− (reactions 7 and 8), but these two reactions are constrained because of the acidic property of our catalytic system. The pH of the aqueous AF dye solution was measured with an Ohaus pH meter (Starter 3C) and found to change from an initial value of 3.57 to the ultimate value of 5.29, 9613
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cm−1 (Figure S8b, Supporting Information) correspond to C C stretching vibrations and the asymmetric CH deformation in CH3, respectively, confirming the presence of smallmolecular unsaturated compounds in the degradation products. Thus, FTIR analysis confirmed that the parent dye molecule was degraded into small-molecule compounds. However, further determination of the degradation products is still under way.
indicating the always-acidic nature of the catalytic system. Researchers28,29 have reported that the hydrogen atoms of the OH bonds in H2O2 easily form hydrogen bonds with highly electronegative atoms (O, N, or F atoms) because of the polarity arrangement of atoms in H2O2. Therefore, H2O2 will be easily adsorbed on oxide surfaces through H-bonding interactions and, thereupon, decomposed into •OH radicals. Based on the above viewpoints, the process of degradation could be described as follows: AF dye and H2O2 molecules are first adsorbed on the catalyst surfaces by electrostatic interaction or H-bonding interactions; then, the •OH radicals generated by the decomposition of H2O2 attack the positions of highest electron density in the AF molecules,30 as illustrated graphically in Figure 12. After constant attacks by the •OH
4. CONCLUSIONS In summary, well-aligned CuO/MnO2 nanorod array films with average diameters of 400−500 nm and lengths of 5−10 μm have been successfully prepared by outward coating of MnO2 on Cu(OH)2 nanorod arrays based on impregnation and heat post-treatment. The characterization results demonstrated that a layer of MnO2 was uniformly produced on the surface of the nanorods. The experimental results demonstrated that the obtained CuO/MnO2 nanorod array films exhibited efficient catalytic performance, superior to that of bare CuO nanorod array films, for the degradation of aqueous AF dye solution in the presence of H2O2, because of the synergistic effect of CuO and MnO2. The effects of the initial concentration of the aqueous AF solution and the H2O2 dosage on the catalytic performance were evaluated, indicating that the removal ratio of AF dye at 20 mg/L was superior to that at other concentrations and that dilute H2O2 solution is economical for the highly efficient removal of organics from wastewater with nanorod array film catalysts. The CuO/MnO2 nanorod array film catalysts were found to exhibit excellent and stable catalytic activity in scaled-up systems, with an AF degradation ratio in life-cycle performance varying from 94.05% in the first run to 90.28% in the 10th run, demonstrating excellent cycle performance and a potential engineering application of CuO/ MnO2 nanorod array films immobilized on copper substrates for wastewater treatment.
Figure 12. Schematic diagram of the catalytic degradation mechanism of AF molecules on CuO/MnO2 catalyst (δ− represents negative charge).
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radicals, the electron distribution of AF molecules is destroyed, and the AF molecules are degraded into small-molecule compounds, as confirmed by FTIR analysis in Figure S8 (Supporting Information). The FTIR spectra of AF dye and the degradation products are depicted in Figure S8 (Supporting Information). The FTIR spectrum of AF is presented in Figure S8a (Supporting Information), indicating several important absorption bands. The peaks at 3444 and 3354 cm−1 are associated with the N H antisymmetric stretching vibration, and the peak at 3236 cm−1 corresponds to the NH stretching vibration. The specific peaks at 1627, 1558, 1540, and 1434 cm−1 represent the CC stretching vibration of benzene rings. The peaks at 1368 and 1300 cm−1 correspond to CN and CN stretching vibrations of aromatic amines. It can be seen that the peaks for benzene rings disappear in the degradation products (Figure S8b, Supporting Information), indicating the fracture of the benzene rings. Bands at around 1191 and 1044 cm−1 (Figure S8a, Supporting Information) are attributed to sulfonate salts. The presence of SO stretching vibrations can be implied by observation of the band at 1134 cm−1 (Figure S8a, Supporting Information). These peaks are also displayed in Figure S8b (Supporting Information) (1222 and 1094 cm−1), verifying the presence of sulfonate salts in the degradation products. The OH stretching vibrations appeared as a broad band centered at around 3244 cm−1 in Figure S8b (Supporting Information), which might be due to the presence of moisture absorbed during the preparation of the tablet. Peaks at 1673 and 1419
ASSOCIATED CONTENT
* Supporting Information S
EDS image of CuO/MnO2 catalyst, XRD pattern and TEM image of pulverous MnO2 nanorod catalyst, UV−vis absorption spectra of aqueous AF solution in the presence and absence of H2O2, linear calibration curve for a series of standard aqueous AF solutions, schematic illustration of the scaled-up catalytic oxidation system, and FTIR spectra of AF dye and degradation products. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*Tel.: +86 551 62901450. Fax: +86 551 62901450. E-mail:
[email protected]. *Tel.: +86 551 62901450. Fax: +86 551 62901450. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the financial support of the National Natural Science Foundation of China (NSFC Grants 20976033, 21176054, and 21271058), the Fundamental Research Funds for the Central Universities 9614
dx.doi.org/10.1021/ie500358p | Ind. Eng. Chem. Res. 2014, 53, 9608−9615
Industrial & Engineering Chemistry Research
Article
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(2010HGZY0012), and the Education Department of Anhui Provincial Government (TD200702).
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